Variable Displacement Piston Pump Failure Analysis How it works and How to Repair

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This Weeks Video features a Nachi Twin Discharge Variable Displacement Piston Pump removed from a Hitachi EX40 Excavator.
We show the cause of the failure and the subsequent damage caused.
We briefly describe How a Variable Displacement Piston Pump Works or operates and we then show you the piston pump assembly procedure for a typical Piston Pump Repair and overhaul using new parts including drive shaft, bearings, complete rotating group (Barrel and pistons) as well as new hardened Steel Swash Plate and Bronze Alloy Valve Plate.
Time Stamps
0:00 Introduction
0:45 The Cause of the Damage
2:00 Change of Displacement Explained
2:55 Twin Discharge Barrel Explained
5:05 Time-Lapse of Assembly
10:20 Assembly Complete.
For a better explanation of suction and pressure generation of piston pumps please see
• Swash Plate Piston Pum...
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The machine's owner contacted us and advised us that the excavator had lost power was very noisy and had a bad vibration. So before removing the pump we decided to travel out to site and perform a flow and pressure test on the pump.
The pump's two outlet ports only generated 10 and 15 liters per minute respectively and could not generate more than 500 PSI (35 bar) pressure from each discharge port.
Based on the flow test results we decided to remove the pump from the machine and bring it back to the shop for a strip down and investigation.
After dismantling and stripping down the Variable Displacement Piston Pump we discovered that the ball end of one of the 10 Pistons had failed and had broken off and dropped the bronze alloy piston slipper into the pump's case.
This caused some of the broken parts to be sucked in by the rest of the pistons causing abrasion and wear to the remaining piston slippers valve plate and swashplate as well an imbalance to the piston barrel (rotating assembly) creating vibrations.
Dylan Loo Maurelli, for Video and Editing
Variable Displacement Piston Pump a Hitachi EX40 Excavator.
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Опубликовано:

5 авг 2024

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Комментарии : 53

@srdoberman 3 года назад

Man ...that was an Excellent video. I appreciate your effort and time in making that . Thanks so much! Could you tell me how the squash plate is controlled.

@Hydraulic.Online 3 года назад

Hello, Jim many thanks for your positive comments, and great question by the way. Firstly, The pump's Swashplate and the resulting variable displacement is not controlled by the machine's operator. The Swashplate is adjusted and controlled by an internal system known as the "Horsepower Control" (HP Control) This pump is actually designed to pump a certain volume of oil per revolution, higher engine speed will result in higher flow up to and until a certain predetermined pressure is reached. The HP Control then takes over and it will slowly de-stroke the pump by moving the swashplate to produce less and less flow per revolution. This de-stroking will be maintained as long as high pressure is maintained eg when forcing an excavator's bucket into the earth. As pressure is reduced, for example on an excavator when you reach out forward to start another digging pass the swash plate springs will re-stroke the pump and flow will increase. If you rewatch the video between 8:20 and 8:30 you will notice a small hole under the valve plate, that hole is fed by an even smaller hole drilled in the valve plate, this smaller hole bleeds high-pressure oil into an internal oil gallery that actuates the HP Control Piston. From 8:36 to 8:46 the HP Control Piston is test fitted for smooth movement. From 9:00 to 9:04 you will see me remove the HP Control Piston out through the back of the pump. The HP control piston is finally fitted from the back of the pump and covered by the strong thick cover plate marked "4020" see it at 10:24 to 10:27 The HP Control circuit does a few things. It prevents the engine from stalling, It maintains a certain high pressure for optimum digging performance. At High pressures, it reduces the flow and thus protects the pump from excessive internal component stresses. Hope this all makes sense. Cheers Minos

@srdoberman 3 года назад

You sir a natural teacher . Thank you for your kind response as I get started on my hydraulic studies. I count myself very lucky to have come across your site. I am checking It out. My Best.

@richsrdschuster3716 2 года назад

Best explanation yet on how it works!

@Hydraulic.Online Год назад

Thank you Richsrd Glad you think so!

@philipknight7180 2 месяца назад

awesome detail, thanks!

@Hydraulic.Online 2 месяца назад

Glad you like it!

@adrianfuentes9408 3 года назад

Excellent video!

@Hydraulic.Online 3 года назад

Thanks Adrian, Glad you liked it!

@joseluizth 2 года назад

Excellent thanks for sharing

@Hydraulic.Online Год назад

Thanks for watching!

@MrCrabbing 2 года назад

Thanks for the video, I have one similar fitted to my trawler, it leaks out of the control rod which I think is the part that alters the swash plate, when the lever is right in one way I have full flow and when its the other way I have next to no flow its a bit bigger than that one but it works very well except for the leak I was trying to see if I could repair it

@Hydraulic.Online Год назад

Glad it helped

@ashiqali-1987 2 года назад

Nice informative video.. a question I have a pvd-3b hydraulic pump one you fixed the swash plate at an angle the machine starts working.. the outlet pressure on the servo piston isn't letting the swash plate move..any suggestions why the servo piston isn't draining the oil ?

@Hydraulic.Online 2 года назад

Have you changed or replaced any parts?

@EvilRace 2 года назад

Very clear steps, thank you. I have piston pump with a huge loss power so it can't move the wheels of a skid steer loader. I checked the piston shoes and they all look ok. I want to avoid buying the full rotating group that it is expensive. I was thinking on these steps: 1. Buying a micrometer and checked diameters of each pistons, if I'm lucky I could identify the bad ones and replace them. 2. Checking the holes diameter of the cylinder and replace if they are not all the same. But after these easy checks, what else could I check? What would be next most likely issue according to your experience if I have to guess a component to replace? Thank you.

@Hydraulic.Online 2 года назад

Check the condition of the Piston Shoes on the end of each piston they should not be burred or bruised and the flat seat has to be clean and scratch-free. The shoes should swivel but not have any end float at all, then check the valve plate and lap the valve plate on a piece of Glass with 1200 wet and dry sanding paper and light oil or diesel to remove all scratch marks. Lap in a figure 8 movement keep even pressure on the plate and rotate the plate every 20 seconds by 90 degrees.

@EvilRace 2 года назад

@@Hydraulic.Online Thank you very much for your help. It is hard to find somebody sharing knowledge in this field. I will probably have to replace the pistons and valve plate. Do you sell spare parts for Cessna 70412-307C?

@Luqmananto Год назад

nice explanation! Recently I had a case where the piston shoe was damaged due to friction with the swashplate even though there was a sufficient amount of oil. What do you think are the aspects that I need to pay attention to in this case? Does cavitation also affect the damage to the piston shoes? Do you have any references or literature that I can read to help me solve this problem? Thank You

@Hydraulic.Online 11 месяцев назад

Bagus Having plenty of oil is only one of many requirements, the oil tank must be allowed to breathe check filler breather, the oil must be very clean check oil return filter if you have had a pump failure you must flush all the oil and change all the filters, the oil must be of the correct type and viscocity for the environment and the machine it is used in , the oil must be allowed to flow freely to the pump, (check the suction strainer this will cause cavitation if it is blocked, and finally the oil must be at the correct temperature, not too cold or too hot. Very Cold weather may cause cavitation on start up, very hot wether will cause accelerated wear unless you control temperature with an oil coolers. As for Cavitation the piston shoes and valve plate will be the first to fail as they are made of the softest materials.

@user-pf3lc8zf1z 2 года назад

hey i was wondering what is the cylinder block for? making a presentation on the pump but cant find whst the block is.

@Hydraulic.Online 2 года назад

If I have understood you correctly the cylinder block It is just a piece of thick wall Aluminium tube that supports the pump housing with stability during assembly, it also keeps the input shaft from touching the work bench and allows it to rotate freely.

@Mi5h0k 2 года назад

hello i need help. is it possible to make a pvd-2b-42 from a pvd-2b-36 pump? what spare parts need to be replaced? Thanks.

@Hydraulic.Online 2 года назад

I think it would be possible with a new rotating group, ie new Pistons new Barrel and possibly a new valve plate as well .....that been said, this larger group will give you more flow or speed but you will need to ensure you have enough Horsepower other wise you may not be able to generate as high a pressure as the 36.

@farmerrick9660 Год назад

Hi, great video, I have a small skid steer I just restored all oil and hydraulic lines are new, cleaned out both chain case reservoirs , everything runs and works great but I find both the variable pumps are really loud, no cavitation, charge pressure is good, they both have great power, just loud hydraulic noise, im not sure if they were built like this in 99 or something needs work inside?, any help would be great

@Hydraulic.Online Год назад

Hi Rick there are many reasons for the noise, pump vibration and harmonics can amplified through the machine's frame, older pump designs, may be less smooth and efficient than more modern equipment, larger clearances in your pump and motor bearings, More internal oil slipage due to larger clearances as wear increases, Oil Temperature and viscocity.

@farmerrick9660 Год назад

@@Hydraulic.Online thanks for the info, sounds like many to diagnose for sure, I’m anxious to take the pumps apart but need the machine desperately on the farm, my main concern is do you think I can do any damage with how it is?, the whole hydraulic system was cleaned out with all new lines filters and top synthetic oil, oil looks very clear even when hot

@rolanromano9035 Год назад

Is there any available market for those valve plate, bearings and piston slipper?

@Hydraulic.Online 11 месяцев назад

Try Alibaba there are many companies that manufacture parts.

@eugeneedwardlumanlan7975 Год назад

we bought nachi replacement pump for our zaxis30, hydraulic doesnt flow on the selector hose when turned on but flows when its turned off. can you provide insight on what's causing this? mechanic is suggesting that we open the pump to see what's the cause of the problem, but we refused as it would void the warranty. can you help ?

@Hydraulic.Online 11 месяцев назад

Not sure what you mean by selector hose. Please expand.

@andrewarmstrong9194 7 месяцев назад

Hello, i have a jcb skid steer with a saeur danfoss series 40 m35 pump My question is i was operating the machine for about 3 hours everything was working normally, then something in the machine caused it to jerk and shut the machine down, now when operating for like 20 minutes or so it works normal, then after that when everything starts to earm up the machine hydraulics get very weak and slow( will barely drive or turn, boom lifts slow) any ideas what i might be looking at? Have you ever heard of a complaint like that?

@Hydraulic.Online 6 месяцев назад

Thanks for the message Andrew we have heard of this issue many times. There are no short cuts in fault finding hydraulic systems, they nearly always a logical process of elimination . The First things I would do is as follows, please try and do them in the order written below as they are all interrelated. Check your hydraulic oil level and top up tank if it is low, Check the type of oil you are using and if it is the correct grade for the conditions you are operating in. Next you will need to check oil temperatures. The best way to check oil temperatures is by investing in a Infra red Digital temperature indicator they are about $30 these days. They are also very handy for using around a mechanical work shop. Check the temperature of the oil when things start to play up, if the oil temperature is higher than 70 degrees Celcius after exiting your oil cooler, then check if the cooling fan is working, If cooling fan is working, then check to make sure your oil cooler fins are not blocked with dust sand and or mud if yes carefully clean out with compressed air and running water. Point the thermometer at the oil fittings exiting the pump as well as the oil cooler inlet and outlet fittings there should be a temperature drop across the oil cooler. If all of the above proves to be Okay then a Pump Flow against Pressure Test is required to check the condition of the Pump.

@stoicahoratiu7540 2 года назад

Hello Sir, I have a nachi pvd 2b 44 pump that is worn out and I ordered new parts for it. It is clear that it was rebuilt in the past and I found something that raised some question marks. The oil passage coming from the small hole in the pressure plate is blocked (plugged) right before it exits the main body of the pump going into the HP control. The HP Control has two pistons, one big and one small. The plugged passage should lead to the big piston. The passage from the small piston is going through the main body of the pump and reaches to the gear pump . My questions are: is this passage supposed to be plugged? and can you explain how the HP control is working with two pistons and in connection with the gear pump (pilot pump)? Thank you

@Hydraulic.Online 2 года назад

Interesting... let me ask you two questions first, Did the old valve/pressure plate you replaced have a small hole as well and is there an external port on the pump that connects to the big piston on the HP Control?

@horatiustoica3832 2 года назад

@@Hydraulic.Online hello, thank you for your reply. the old pressure plate that I am replacing has a small hole in it, but like I mentioned, this pump was rebuilt in the past so I have no idea if the original pressure plate had this hole (I guess it did). There is no external port between the pump and HP control. Meanwhile I managed to extract the "plug". It turns out that it is a screw plug that has a tiny orifice drilled into it(which was clogged). I measured the hole in the pressure plate and it is 0.65mm in diameter and the hole in the plug is 0.5mm. So it looks like a flow/pressure restrictor. My guess is that this orifice being clogged, lead to the excessive ware of the pump. The issue is that the process of extracting the screw plug, rendered it unusable. The question is: is this a vital part for the pump or it can work without it? From what I can tell, it is original and not some modification

@marcomarcasunocc2364 9 месяцев назад

Hello friend, a question Is the swing plate damaged? since it has a slot or is its design like that because the times I have had to check the pumps I have not seen swingarms with a slot and if they come I will change them for a new one without a slot I am from Peru greetings

@Hydraulic.Online 8 месяцев назад

Need to see some pictures of the item you are talking about.

@smoke_stackz3168 Год назад

How to adjust it my pump has un equal presure on p1 and p2

@Hydraulic.Online Год назад

Depends on the design and if it has separate relief valves.

@santassantas9395 2 года назад

hello Mr, I am an electrical engineer, I have a hydraulic drilling machine. The hydraulic oil has been heating up for a while and the pump too yet the entire hydraulic control system is responding well. I tried the bleed, I cleaned the hydraulic radiator, checked if the blower and even bleed the pump but nothing. all are functioning properly, but it is the pump and the hydraulic oil that heat up. what to do mr because i'm really stuck. I need help

@Hydraulic.Online 2 года назад

Best thing to do Santas it to get an infra-red temperature indicator and check for hot spots all over your hydraulic system. How hot is it ? Temperatures up to 80 degrees C at the pump is OK as long as the oil in the tank is below 65 degrees C. You may have an over pressure or flow control issue and oil is going through a pressure relief valve in the pump.

@santassantas9395 2 года назад

@@Hydraulic.Online okey thank you sir

@javidbepari8704 9 месяцев назад

I facing problem with Nachi make hydraulic pump. P1 work good but P2 not create 135bar . P1 generates good pressure upto 195bar. What's the problem??? Please help.

@javidbepari8704 9 месяцев назад

P2 not generate above 135bar

@Hydraulic.Online 8 месяцев назад

You will have to strip the pump down and make an inspection probably a damaged valve plate, piston or piston slipper or a suction issue.

@joosharmse7606 Месяц назад

Forward and backwards is going don't want to going up hill it stop

@estebanmalan9627 2 года назад

Algún manual de fallas en español

@Hydraulic.Online 2 года назад

Debería poder copiar o descargar la transcripción de mi video y usar el traductor de Google para convertirlo al español. You should be able to copy and or download my video transcript and use google translate to convert it into Spanish