Mrs. AvE: Honey, the shower is leaking. Mr. AvE: Fear not my sweet, I'll fix the dibbly doo with these spare Orbitrol valve parts to corral them escaping melting icebergs. Mrs. AvE: Do I have to wash my hair in hydraulic fluid again?
Holllyyyyy... Yeah, I mean that pitting, its just part of the industry. A lot of times when the contracts are called, especially from government contracts; they order X amount, then take Y amount and tell you to inventory the rest. A few years go by, that part now becomes obsolete. They just sit on a shelf, in a box, in packing material. As the years go by, the oils start to break down. My pop had a specialty grinding shop and after 60 years, we closed it down. The amount of stock we had was mind boggling. Plus after looking at the inventory, a lot of it had oxidation and pitting. Even though they were sitting in boxes and were oiled before storage. When I look at parts like that, it makes me cringe. Just knowing the guy who made that part probably was pulling his hair out. With such a tight tolerance, probably a class 3 micro finish, plus it going through heat treating and such... When the machinist saw that order come to his hands, he was probably MFing the engineer who designed it on the computer. haha. After we closed, all of our contracts went to our competitors, they have been calling ever since to ask for our operations sheets on how we manufactured these parts. 40 years of hands on experience always beats the engerneerd with that paper that says "Masters" or "PE" Like the vijayos. My pop always gets a kick out of them.
Give this man a medal, for actually making sense out of the comments of his videos and using it as feedback in the next one. The amount of dedication that takes, boggles my mind.
Thank you for having a bucket full of testicular elements that allows you to correct when you make errors. Wish this was more common in the world today. Knowledge, ability, and morals are things not readily packed into the same people anymore. Bravo good sir!
The spring provides feedback to the driver (thats the outer loop on the main hydraulic valve body on the diagram). It does that by giving some return force between the inner and outer shells of the spool. The pin that came out in the first video is your backup on failure, as that allows you to hit the limit on the spool valve and then drive the spline at the bottom via pin. The hydraulic rotary pump you were discussing in the last video is attached to the shaft, so there's a complete drive path from end to end of the power steering system. That means that if you don't have a working pump you're driving the shaft through the centre of the pump via the dowel pin that was through the centre of the valve body. They're a rotary proportional hydraulic valve and a hydraulic pump.
Hey great work on the explaining of this complicated valve. Just one thing. Yes that motor/pump definitely does act as a pump in the event that you lose supply pressure and you have to tow it or such things. Have had to do it to some asphalt rollers and whatnot over here in Saskatchewan! Absolutely love the technology and precision of these components. Thanks for the great vids!
As I millwright and a certified industrial electrician I have enjoyed all the stuff you do and explain. Would like to see you dive into a PLC. I have lots of experience with the Mitsubishi ones. Could use it to tie all this stuff you have been showing us and make a rue Goldberg device! That would be awesome!!! Also good for others to see how kinda simple the PLC’s can be... just a thought 💭 great as always
I'll disagree on your assessment of the steering without hydraulic power. I have two pieces of equipment with orbital steering. You can steer without the pump running. Yes, it is steering by "Armstrong" but you can steer. It's an NHTSA and OSHA requirement for operator safety.
Back in high-pressure fuel injection pump school, those tolerances were referred to as "class fit"- Classy indeed, I saw a tool from the process with the label, " Not plus or minus a thousandth, but exactly."- From the master-mind Vernon Roosa, inventor of the calibrated pop can, the Roosa master pump, an amazing piece of thought and engineering that even after studying and working with for 25 years, still boggles my mind how someone could dream this up.
For precise air cylinder positioning you need 4 air valves. You keep both sides of the piston at the same air pressure,each side has a intake and exhaust valve. To move piston ,intake valve from one side opens in pulses and the exhaust valve from the other side opens in same pulses,piston then gets moved in a VERY precise matter. Add a mechanistic pulse strip and sensor to the arm and you now have the perfect robot arm,strong and holding any position solid.
As far as the air over hydraulic/electric over hydraulic thing: I drive commercial vehicles for a solid waste hauler on occasion (Backup driver, normally I'm the IT guy) and their systems are set up in a few different ways. For the type of application that requires the hydraulics to operate in a binary mode (full flow on, or off) such as the automated style with the arms that reach out to grab the can, those are set up so that the buttons inside the cab are electrical switches that actuate air solenoid valves. The air solenoids feed truck brake air pressure to a pneumatic servo attached to the hydraulic spool valve to shuttle it back and forth. (I call those electric over air over hydraulic. Electrical system controls the air system, air system controls the hydraulic system.) For the systems that involve more finesse, the drivers can use air valves that work more like regulators, where pulling the lever back halfway causes enough pressure at the pneumatic actuator on the spool valve to shuttle halfway through it's stroke within the valve body, this gives the drivers a degree of control much closer to what they would expect if they were manually actuating the flow control lever at the hydraulic valve itself. We use these on our Front End Loader trucks and our Roll-Off style trucks. For our own purposes for Roll-Off trucks we use a spool valve that accepts both manual input via a lever (pure hydraulic) as well as air over hydraulic from a control valve within the cab. The spool valves look very similar to this: i.imgur.com/N5BfI6p.jpg the manual valve on the left, and the air control is on the right. Either of those two systems can shuttle the spool valve within the valve body. Hope that helps, AvE! I could provide more pictures but our trucks are all out in service currently.
Hey AvE, maybe the one I have works slightly different but I drive a tracked vehicle with a hydraulic steering system (Hägglunds Bv206) that DOES in fact let you turn the wheel to pump hydraulic fluid and steer the vehicle when the hydraulic pressure from the engine is nil. It takes a whole lotta turning but it's doable, I've done it several times.
Very true statement the internal pump is assisted by the external It is not just for feed back When external pump fails the internal pump becomes an emergency pump and can turn the wheels if you apply enough force on the steer wheel It also takes more turns of the steering wheel when in emergency mode because of slippage in the internal pump But you can steer it
Have a Yale Forklift.....SAME WAY. Spin the steering wheel with engine off, and eventually the wheels turn. So YES it does work as a pump, and it's designed to work that way.
I have relocated a mil spec 10k forklift that was secured. (padlock and cable on stering wheel)just had to kill the motor and take another bight refire the motor turn some more, repeet 1/4 turn increase each (8 starts lock to lock)
Agreed, I’ve done it on tractors and dumpers with dead engines, it’s a whole lot of winding away on the steering wheel, and it generally feels like the pressure is leaking away as you go but they do turn....
Example of electric over hydraulic: Older heavy equipment used levers directly attached to the valve bodies to actuate the hydraulics. Now with electric over hydraulic, the valve bodies are controlled by solenoids and then can be move to more practical locations. Maybe for easier access, or lower center of gravity. Also, by moving the valve bodies, you get less heat and noise inside the cab, and move the high pressure hoses further away from the operator. (Safety). Finally, you can get more functions crammed into a joystick and minimize repetitive motion injuries. Added bonus, I can adjust the sensitivity of the controls, the speed of the movements and program for repetitive motions, like returning the bucket back to a dig position. Thanks for your hard work on these videos. I always look forward to the next one!
The way I understand “air over hydraulic” or “electric over hydraulic” is that an air or electronic actuator is used to change the position of the directional control valve (what makes the fluid go to different directions to make cylinders or motors move.) The reason for this is that sometimes the pressure from the system is so great that it would be too much to actuate by hand. Sometimes it’s due to location of the valves (think relay) it’s easier to run air or electric lines to the operator position instead of hydraulic lines. Hope this helps!
I think ya maybe kinda missed one. Now I'm no expert, but after minutes of in-depth superficial research it seems to me that the gerotor is NOT only providing haptic feedback, but providing feedback to the valve to provide mechanical resistance to some arrangement which allows the inner component to rotate, the width of a groove or so, independently of the sleeve, until the whole thing turns inside the housing. It replaces the mechanical connection of, say, an automotive rack and pinion, for example. In the automotive boxes, the sleeve is connected to the pinion with an indexing notch, but the inner element is connected to the pinion with a torsion rod. The torsion rod is part of the springiness you feel when you try to move the wheel without the engine running. When you begin your turn, the mechanical resistance at the pinion provides the anchor point for the torsion rod against which the steering wheel first acts. There is also mechanical lash limiting, so it's not like you can just wind the thing up. As the rod twists it allows the inner element to turn say 1/8th of an inch while the sleeve is prevented from rotating until the rack actually starts moving. This opens the appropriate port and pressurized juice is sent to where it's supposed to go. When you stop turning the wheel, the torsion bar returns the penis to the neutral position inside the pussy, and the juice just runs in circles. Wheel shocks likewise bounce off the torsion bar and are automagically compensated for by the valve. At least, that's how I understand it. Which begs the question: how does this happen inside the Orbitrol? If the gerotor pumps to the cylinders, it must pump essentially nowhere until the ports open, for instance. Cheers!
I asked about these not long ago! Thank you so much! Makes sense! I’ve heard off road guys say they can get some useful movement in the steering with a orbital valve even in the event of a power steering pump failure. But I’ve never played with one myself.
Sorry but you still got it wrong. Reading the schematic, you can see that the valve has 3 states (indicated by the 3 devisions in the block) between which it transitions smoothly (proportional). The left state of the valve connects the pressure to the right side of the manual pump, the left side of the pump gets connected to the left cylinder, and the right cylinder gets connected to tank. The right state of the valve connects the left side of the manual pump to pressure, the right side of the pump to the right cylinder and the left cylinder to tank. The center state closes both ports of the manual pump. Now the smart part: The entire proportional consists solely of those two cylinders, so the valve rotates with the steering and it is actuated by relative movement between the two parts. The inner part gets driven by the driver and the outer part gets driven by the spring elements. The outer part in turn drives the manual pump, this makes the valve a torque sensing control system. So when the driver applies torque, the valve applies hydraulic pressure to the pump inlet and assists the driver this way. (when no torque is applied the pump is blocked and therefore the outer sleeve. This forces the valve to operate as soon as torque is applied). The consequence: even with no pressure you could operate the steering, but it would require a lot of torque. Cheers from Germany.
just to touch on the portion of the video where you spoke about pneumatic cylinder seal friction. Most pneumatic cylinder manufacturers, especially those manufacturing NFPA & ISO Cylinders, offer a Low Friction/Low Speed Mod, where they put different piston seals and/or a special lubricant into the cylinder so the cylinder can be controlled more precisely, or so that the piece of equipment it is attached to can act upon the cylinder with less force. So what you said about the more load you put on the cylinder, the more accurate your positioning, 100% correct because yes the friction, proportionately speaking, is less with a load on the cylinder, but again, for applications with little to no load, the low friction/low speed mod really helps out. Cylinders with this mod are also good for shock absorption. I have a customer who actually uses some 2.5" Bore x 4" Stroke Cylinders with this mod on a machine where they hold some large wire wheels to clean drill pipe, and the brush heads need to be able to easily move up and down an inch or so when the tool joint of the drill pipe passes under the brush head.
The crosshatch you see there is so that those surfaces would retain some Shmoo...at least that's what it would most likely be for in a similar part in the aerospace industry (some customers will actually reject parts for being too "smooth")
Regarding cavitation, the material removal is caused by the sudden inrush of fluid into the imploding bubble/void. So rather than being sucked off, so to speak, its actually getting blown off (my personal preference) like a pressure washer. Worth a sticker?
that adjuster with the spring is the pressure relief, i used to set up the hydraulics on new agricultural machinery and you would take out the plug with the copper washer and adjust the relief until you had 1000 psi at the front axle steering ram
The pump is there for a few reasons. It does allow for some emergency steering in case of engine failure. It also provides some feedback to the operator. Most importantly, it meters a known quantity of oil to the steering cylinders for every turn of the wheel. It's sized to match whatever the flow requirements are for the desired steering speed.
That inner rotor is not ground, it is a powder metal part that was pressed. The sizing/restriking operation after sintering will produce very tight tolerance parts with an excellent surface finish. The "corrosion" is porosity and is characteristic of powder metallurgy.
The fit on the valve is the best fit I've ever experienced! You can tell by dropping it in on a level surface and watching how slowly it moves downwards. Nothing cooler than a mechanical proportional system. You should look into a danfoss flow amplifier if you want to see technology at its finest!
I'm interested to see that Sumitomo Cyclo Reducer because I do maintenance at a facility that uses the Sumitomo Cyclo 6000 series reducers everywhere and we've been having them fail pretty consistently. Some are grease filled and some are oil filled. We're going to be sending one out to get autopsied. We know a grease filled reducer is going to fail once we start seeing external temperatures above 110°F on the outside of the housing.
Damned the automatic translation of your shop is killing me: WICHTIG! 4 Wochen zu versenden, dann jedoch lange Ihre Mailman braucht, um seine Sache zu tun. (Die Familie Sweatshop braucht Zeit, um Chooch.) Sie erhalten: 2 Aufkleber von unserem Lieblings-Pin up-Modell; die klatschte, ' Ol Bridgeport Fräsmaschine (nicht geeignet für Ihre Mutter-Bridge-Club) 4 "x 4"; 100 mm x 100 mm, gestanzte, hochwertige Aufkleber
When I see "air over hydraulic" it usually means they are using air pressure to move the shuttle or shuttle back and forth in the spool valve. "Electric over hydraulic" is usually using 0-10V or 4-20mA to run an actuator to do the same.
Takes a big man to admit mistakes on camera, but I'm glad you did and you rectumfried it for us. Your vids are awesome ave and I have learned a lot from them, and the commentary is killer lol. Thanks for sharing your knowledge with us common folk. Have a good day eh.
So if the steering wheel is dead center, but the rig's wheels are currently pointing (say) to the right, how does the valve a) know that and b) accordingly allow oil to flow to the cylinder that'll act to straighten out the wheels? Because if it were simply a proportional valve connected to the steering wheel (as still seems to be implied in this video), steering wheel dead center --> proportional valve at dead center --> no oil flows --> the rig's wheels stay pointing whatever direction they were pointing before... which would be NFG. Is there a springamathing connecting the hydraulic pump/motor within this widget to the valve? Because if so, the proportional valve's position would actually be halfway between the hydraulic motor's position (which reflects the current position of the rig's wheels, as all the oil that travelled to the pistons went via that motor), and the steering wheel position. If the motor is running backwards, that "halfway" becomes a "difference", and hey, that's the actual explanation of how the valve works -- it calculates the *difference* in the position of the rig's wheels and the steering wheel, and acts accordingly.
Generally speaking, the whole this over that means that the top mechanism powers the lower mechanism. In other words, snow plows around here are electric over hydraulic. In this specific case, an electric motor/pump uses hydraulics to move the blade. It is about power vs actuation.
You know for those of us who don't read hydraulic schematics (or whatever you call them) it'd be nice if you start out with the connection lines first and when/where they are connected & when/where they crossover and that sort of thing.
I would attempt to criticize you but I am not even a freshly minted enginerd. I'm only 16 but the info is good enough for me. I love the technical teardowns but most of all my 16 year old ears love your vernacular. Keep up the great and interesting work.
The Western snow plows(and other brands) on my trucks are electric over hydraulic. The electric switches in the cab control relays that control the pump and the valves that control the cylinders. My old Western plows were mechanical/electrical over hydraulic. It used a switch in a mechanical joystick and cables to activate the pump and control the valves for the cylinders.
7 лет назад
The blue print; I’m not positive but I think: if the solenoid is left or right it directs flow to what your calling a pump/motor but what I suspect is the proportional valve. So when left or right is hit on what I would guess is a joystick the solenoid shifts left or right and the proportioning valve rotates controling flow rate as per the joystick. The 2 relief valves at the top allow relief for when the cylinders are fully extended. The bottom pressure relief seems to be for when the solenoid is shifted left or right but after the steering cylinder is maxed out this pressure relief (probably set to a lower pressure than the two above) is opened and leads to a limit switch that might center the solenoid or open a bypass or turn off the pump. Love the videos👍🏻 keep up the dicking around, it’s better than me having do it.
It can work as a pump. Had a combine harvester shit out a valve way back in a corn field and had to tow it out to the road for a new engine. Took ~50-75 lbf/ft on the wheel but it could be steered. This is the same type of steering actuator most farm equipment uses.
Hey, AvE. Thank you for the video. Always a joy to watch you take apart the guts of some common used but obscure mechanical mechanism. If you would allow me to ask a favor, use some layout "fluid" (Fastenal) red or blue as a witness mark. You must take enough hits from folks. (I won't add the brand name WHICH would add another word for them to misconstrue to your vocab.)Take a second and wait for it to dry so up and down would be so much easier to re-assemble. Thx Again Friend. P.S. You better start using your John Henry and Heart properly. To tinker is sublime, to love and be loved is divine.
Air over oil: pneumatic power, but flow controlled hydraulically. Industries always have compressed air and since they already paid to have this air pressure, it's cheaper to put that power to use in air over oil, cylinder for example, than to have an independent hydraulic pump. A large diameter air cylinder provides plenty of skookum power but air is a shitty medium for precise speed control. Hydraulic oil pressurized from the air piston gets metered through check valves and solenoids to combine the, already paid for power, with precise flow control. Quite a mouthful, but clever as frig!!!
hey man, just wanted to say I've been following ya for awhile. I enjoy your videos, and I look forward to seeing new uploads. I've been watching so long I'm starting to learn some shit!
Mr. Ave i mean to call you out on another mistake. Yes that little pump inside will create enough pressure and flow to move tractor tires by hand or with out flow from the main pump. How ever it really is power steering by arm strong. I'm a Deere John mechanic out of California and love the videos, a lot of what i know about hydraulics is because of your content keep up the good work.
As my bretheren would agree, you are a fucking class act! You provide shop knowledge to the masses, and are more rounded with experience than most people. As your comment section shows, your contribution to the maker community is greatly appreciated. It is awesome that you can bring a group of 200 lb gorillas together to learn, and more importantly, work together as a whole to use logic working on a hydraulic circuit. Great vidjeo!!! Im eating up this hydraulic shit! I have squirreled away quite the collection of componants, and would like to start making videos helping people learn about schematics, flow controls, pressure controls, and directional controls by tearing them down and putting them back together and eventually, delving into more advanced hydraulics. You have really inspired me, but I have a face for radio, and voices in my head. Wait, what??
AVE, a quick question. My Dad is complaining about the clutch on a tractor that he drives every day, he does hedgecutting, so it involves alot of clutching. i was looking into an electric servo motor to actuate the clutch and just using the pedal as an imput, but im not as level headed with pixies as i am with schmoo. If i could rig up a hydraulic system i would just undersatand it a hell of alot more. If i use one of these orbitrol units and he keeps the clutch half pressed for a bit, will there be oil leakage in the system and then left with the clutch at the floor and no more clutching. as i'm aware some steering units let the steering wheel spin a little bit all the time, I wont be doing that with the clutch..... any opinions on what sort of proportional valve i could use alternatively? love the vids, keep up the good work.
If I'm not mistaking, that system is also used on ordinary tractors. They have hydraulic steering, and yes, you can in fact use the steering as a pump and turn the wheels with the engine off, but it is haaaard, and not something you want happening when driving down the road.
few heavy duty applications use old time green coolant ( comes in many other colors as well ) not extended life which usually only comes in red, except cummins dyes it blue. the difference is fully formulated coolant contains sca's and nitrates to prevent cavitation and silica for more btu carrying capacity. car green coolant only has the silica. no sca's, no nitrates. thats the simple answer gets fairly complicated once you get into extended life coolants, NOAT, OAT , Hybrids, etc etc.
After reading up on it quite a bit, I came to the conclusion that there's no reason to run anything but Fleet Charge. Over time I have slowly drained and flushed every vehicle I have and made the switch. Everything, from my tractors to my '70 Chevelle with a 550hp 454 has Fleet Charge in it now.
I deal with milking robots, and they have air cylinders that can be positioned within ONE MILLIMETER. They are made by Festo (of course). Each cylinder has four valves - an inlet valve and an outlet valve ON EACH SIDE. The cylinder has a magnetic Scale (?) inside of it that is read for positioning. 6.5 bar gives you all the speed and force you need.
That oil pump looking thing at the bottom of the valve body, every time you put that 6 lobe rotor inside it's cavity and turned it, it gave me goosebumps and freaked me out. The sound of the metal scraping reminds me of nails on a chalkboard, fuck!
Air over hydraulics. I always understood it as the use of two different systems to work together or assist one another to accomplish one task. I'm going to use a Military 2 1/2 ton utility truck (Deuce and a half is what it's usually referred to). It has air over hydraulic brakes. The brakes themselves are hydraulic driven. The truck has an air pack on it that essentially senses when you press on the brake pedal and helps you by converting pneumatic power into mechanical power into hydraulic power. I never tore one apart but It'd be pretty neat inside probably. They used this system because the fact that if you got a hole in an air tank, you still have brakes because all the air system does system does is assist the hydraulic. Technically it would actually be an air assist over hydraulic, yadda yadda. You get the point.
You are absolutely correct in your assumptions about air or electric over hydraulic. Much the same as a pilot hydraulic control would be hydraulic over hydraulic, if that makes sense
You are the best thing on internet mate! Air over hydraulic is very similar to smaller rigid.truck brakes. Air is pushing the hydraulic fluid allowing extra "pressure" with out the big pump...... do a video on it: )
AvE that pump is extremely similar to the pumps on light duty tractors. And it is hard, but on a little 80 horse 2 wheel drive tractor, you can turn the wheels with no clangin and a bangin of the motor. Also, you said it was a pump and a motor. This is true. You disconnect a couple lines on the power steering system of a light duty tractor, and you can use the pump and the 300 pound gorilla to completely flush out a power steering system. (Small farm tractors and what not use a power steering pump, separate from the hydraulic system, in conjunction with that hydraulic pump.) but ya know, you take the same setup and put it on an articulated loader weighing in at a whopping 40 tons, and you’ll never turn the thing by gorilla power alone. But the concept is the same. Great video, I️ can’t wait for more hydraulic systems videos to come out. You fuckin killed it like always.
As the saying goes, jack of all trades, master of none. You can't be an expert at everything. I enjoy the videos, some of my safety minded co-workers didn't find the engage safety squints shirt as funny as I did when I bought it
The orbital valve will extend the cylinder without pump pressure under no load conditions, but it takes a 300 pound gorilla. Used on planetary gear reduction axles in military vehicles. Worn orbital valve can cause undesired porting within and cause non demanded steering output. Very interesting for the driver.
There’s no triangle on hydraulic actuation when it is remote, if the pilot is internal then yes there’s a little box with a triangle but there’s no line. Also if there’s a drain for the spring it comes out of the spring, not the valve and it’s usually shown that it goes straight to tank. I know hydraulics aren’t very standardized with the schematics but that’s my two cents.
You can steer sligtly as you said. But it takes ridiculous amount of force and it's more likely to break the steering wheel than to get anything proper. Saved me a hour of sitting in the garage once so I was happy
My understanding is that Air-over-hydraulic systems have an air driven piston creating flow. Electric-over-hydraulic has an electric motor creating flow. Your Hy-torq setup, for example, would be air over hydraulic
I think the Geroter gear is made from powdered metal. I use to work at a PM factory, we made 100's of thousands of gerotors. The pitting looks like normal pm part. Measure the OD of the top and bottom, if there is a .001 difference its probably a pm part.
I live in Germany and as far as I know even if the motor fails you must be able to brake or steer at any time. We have a tractor and if the engine is off and you are standing you are never able to turn the steering wheel, but when you are rolling down a hill it works just fine, quite hard but it does. I think it also uses a similar valve, not sure thought
rahight-rahight-rahight... Skookum. b.t.w. (if ya don't mind sharin) what kind of microm'scope you got there? Lookin to purchase one myself. thanks buddy.
air, hydraulic, electric all have different rules and conventions. when you say X over X (like air over hydraulic) what you really mean is that the rules of the first one (air) apply but it reality its the second one that actually covers the last mile of delivery so to speak. so basically what you said is correct. air over hyrdo lets a hydro break system interact with an air breaking system. kinda like a transducer
New subscriber. Poked around through a few hundred hours of your vids. Good stuff brother. A redux of that pneumatic metering would be awesome. Also it would make me look smart the old candy fucktery.
couldnt believe my eyes when i saw those horrible lux tool pliers on your bench - you get these at Austrias equivalent to Harbor freight! ps: thanks for the stickers that came in today after 6 months of waiting ;)
Hey Uncle .. nutha sweet vid!! Not sure if this the best place to ask such a question But do you have skookum as frig t shirts still? Cheers all the way from NZ
The pump on the bottom is for emergency use if you loose hydraulic pump flow you can still steer the vehicle, you won’t have power steering but you can still steer
I'm not sure if this is an industrial standard or not, but in my shop we have an air over water extrusion press ( at least that's the lingo that has been passed down) and its physically 5000 psi of air, over 2k ish gallons of water. air for the massive volume of stored energy, water so you don't get the " spring-in-sprung " is this how others use this lingo ? or are we bass-akwards ?
They are a hydro-mechanical closed loop system. Move the spool by steering and the flow brings it back to center to close by way of displacement-steering, like in a car. The other type is velocity-steering whereby the wheels would continue to turn until you center the steering input. These units are some of the coolest mechanisms in mechanical engineering. Like you said, pornographic.
When you say "air/electric over hydraulic" you're basically just saying that the valves that control the hydraulic fluid flow are either actuated by air pressure, or electrical signals.
Odd timing that you mention the references to air-over-hydraulic or electric-over-hydraulic as this was quite the discussion on a jobsite last week. I had been taught by my grandfather (mechanical engineer in the 60's) that these were the shorthand lingo to describe air-actuated or electric solenoid actuated hydraulic controls in parker manifolds or valves. To my surprise, these terms got shortened yet again to simply "air-over" or "electric-over" with no mention of hydraulics at all. My guess is that all of these originated in the field or regionally to help mechanics or repair technicians not so versed in hydraulic systems understand why the hell there were wires or air hoses coming out of something that uses hydraulic fluid to do all the work. I guess it exists in the same unwritten jargon as a slapstick valve. But hey, if it chooches, I don't care what term they use...if they break it, I'll have a few choice terms of my own to share with them ;)
