How a Hydraulic Gear Pump Works and How to do a Rotation Change

Ever wondered how a Hydraulic Gear Pump actually works to Pump Oil and create flow and pressure?
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Video Highlights
In this Video, we dismantle an Italian Galtech gear pump and explain the following,
How the oil moves through the pump when the gears are rotated.
How the internal pressures generated can create reaction forces and internal wear marks.
Briefly explain what parts and seals need to be reversed when a rotation change is made.
The location of and function of the seals on the combination port and bearing plates.
This video is part one of a gear pump repair series where we strip and repair the damaged thread on the main housing's Pressure Port.
Dylan Loo Maurelli, for Video and Editing

[@sanjusugathan5321]

Is there any need of changing position bottom swab plate

[@Hydraulic.Online]

Good question Sanju, if you are changing rotation you must also change the Bottom Seal/bearing plate assembly as well, otherwise, there will be too much internal leakage, causing poor pump performance and you also risk blowing out the shaft seal from too much internal pressure.

[@guypatts494]

My gas motor turns ccw so hydraulic pump needs to go cw ?

[@arnoldschmidt2753]

Thank you for the physical demonstration. The place I took my Case 90Xt to ,wouldn't explain anything to me about how the pump either worked or how to fix it locally. They just pressered me into getting a rebuilt / re manufactured pump . They also told me that if I didn't want a re man pu p ,that I could wait 8 to 10 weeks for an new OEM pump. Which I agreed to. I was much happier to go with a new one as opposed to something that might not be 100 %. Then 2 weeks later they fired the mechanic who kept me informed and after that they canceled the deal . So much for trusting people.

[@jamesdepaul3410]

Very good info, thanks very much

[@Hydraulic.Online]

Glad it was helpful!

[@ok_computer12]

This was a great demonstration.

[@Hydraulic.Online]

Thanks Saleel

[@sk1nnygal]

Hi, I have the engine off my log splitter for some other work. I was turning the pump by hand (just fiddling) and noticed it would turn freely then stop (almost like a notch) in the same place once per revolution. I can easily rotate it through the notch though. What would cause that? It was fine all hooked up to the engine last time I used it. Thank you

[@D59RAG59]

Nice

[@Hydraulic.Online]

Thanks

[@kayvonmansouri]

Great video! I need to switch the direction of rotation by switching the gear position, otherwise my engine will turn it backwards. When i do that, does the inlet stay the inlet for the low pressure side, and the outlet stay the outlet, or do those switch as well?

[@Hydraulic.Online]

HI Kevin sorry missed seeing your question. Just remember this rule, if the outside tips of the 2 gears are rotating and then meeting and disappearing through the middle that will be the pressure / discharge port, you should be able to flip the external gear housing so that the smallest port is on the pressure side, the other rule is always have the largest port on the suction side (some gear pumps have suction and discharge ports of the same size then it does not matter)

[@kayvonmansouri]

@@Hydraulic.Online thanks so much

[@kayvonmansouri]

When you reverse it, is there a plug that needs to be moved? My Eaton pump has one. it plugs off a lubrication channel to a top bearing in the cap and it seems if that is on the high pressure side it blows the seal. I moved it to the low pressure side and it stopped doing that. I couldn't find any literature on whether it should be on the high pressure or low pressure side. my concern is does that get lubrication properly now from the low pressure side. Thanks for making the video!

[@Hydraulic.Online]

Hi Kevin good question!, modern pumps have designed out this requirement, I am trying to remember my gear pump theory from technical school but I just can't remember exactly, I will check my notes as I still have my books. From what I remember. The High-pressure side should be blocked off and low pressure or suction left open. This way high-pressure oil is forced from the gears down through the shaft and bearings (for Lubrication and cooling) and into the low-pressure seal area where it is sucked up and recycled by the pump. Please Check your pump carefully and make sure that there are no more grub screws in the hole or that it is a ball-type check valve design. try looking at this video it may help go to 2:39 if the video does not automatically start there kzbin.info/www/bejne/mmKwmGNrq7CEnbs

[@kayvonmansouri]

@@Hydraulic.Online I re-read my post and I wrote this backwards. I closed off the high pressure side and that partially solved my problem. Then re-watched your video and you spoke about the wear plate orientation and how the seals hold back the pressure of the high pressure side. I took mine back apart and sure enough, it was backwards. I figured those seals wouldn't make contact with the gears, so there was only one orientation left. Seals facing away from the gears, and the open end towards low, and the blocked end towards high pressure. I reversed it to the correct orientation and then that pump/tiller combo started acting like it should. And the pump quieted right down. I just wanted to thank you for taking the time to make this video. It helped solve a problem that has been plaguing me for hours and days. FYI i just watched the video that you suggested and yes he's got the high pressure side plugged.

[@thanatos103]

Can one of these simple external great pumps be used as a motor without pushing fluid passed any seals? Do you need to apply pressure in to the output side of the pump to accomplish this, and how does that internal "3" seal need to be oriented in that use case?

[@Hydraulic.Online]

It is not recommended to use a pump as a motor as all the internal pressure and shaft seals are different.

[@creativefireplace3329]

I’ve seen this video and I did understand how a hydraulic pump works..ok. Now I have a problem with a hydraulic circuit (or installation) from a tractor. As following: the pump is moving the cylinder for the 3-point hitch, but the oil doesn’t stay in the pump. When the tractor is facing downwards the oil enters the pump and the installation is working but when the tractor is back to the horizontal line or is facing upwards the oil just doesn’t get to the pump, it flows back in the inlet line and it won’t bring up the cylinder and the 3-point hitch. What I don’t understand is how the oil is feeding to the pump. Isn’t the same principle as in the water pump, where the water is kept under pressure (negative pressure, suction pressure) in the inlet line, or is working differently and the problem is elsewhere. Or could that be that there is not enough oil in the installation? But the oil is at the level point in the feeding oil tank. Any help and advice are welcome.

[@Hydraulic.Online]

Thanks for the message Brfiscu Your problem could be either of these issues so check each carefully one by one. 1. Low oil level. Some tractors have more than one oil tank check carefully all interconecting pipes and hoses. 2. Wrong Oil Viscocity for the temperature (try a thiner oil in the colder winter months) 3. You have a suction leak (check the suction hose condition and the suction hose clamps from the tank to the pump) 4. Your Pump is old and to the internal gear tolerances are too large. 5. Check your in tank suction strainer for contamination, If you have one clean it out or replace it. 6. Check your oil tank for contamination, like a piece of rag or a piece of an old tank lid (we had an intermittant problem on suction that turned out to be a sock drifting around inside the oil tank) 7. Check your oil tank's filler / breather cap is not blocked with dust and particles. Please let us know how you got on.

[@creativefireplace3329]

Thanks for your comprehensive response.. I will check all you have said and get back ASAP.

[@fegales5386]

What would be the cause my hydraulic pump for forklift always blow its oil seal,even i install circlips to hold down the oil seal,thank you.

[@sadamhusein6307]

its because you use wrong rotation pump hint if the "3 shape" like seal is on thrust plate or bushing , you can change rotation , sample like this video if the seal is on top cover , you cant change rotation

[@Hydraulic.Online]

If you have recently overhauled this pump you may have installed some parts incorrectly, also Some Pumps have check valves or plugs and oil galeries to divert high pressure oil from the shaft seal back to the low pressure suction side check that they are not blocked by the new seal or blocked by debris.

[@tapiwamutembwa3670]

Please I need help. Have changed rotation on a PZB gear pump but I keep experiencing shaft seal damage. There is a hole in the pump housing that lubricates the drive gear shaft bush and this hole is now on the high pressure side. And oil pressure keeps damaging seals. The hole is normally on the supply side were oil enters at low pressure. How can I over come this? Can i block hole on pressure side and drill now hole on suction side so that drive gear shaft continues to get lubrication?@hydrauliconlinm

[@Hydraulic.Online]

Yes I am pretty sure you can do this, drill carefully!!!.

[@melikesena788]

which material is used for each part?

[@Hydraulic.Online]

Generally in medium to high pressure Hydraulic Gear pumps, the Gears and Shafts are made from Heat Treated High Carbon Steel. The Casings or pump housing, End Plates and Covers can be Cast Iron, Aluminium Alloy, and or Steel or any Combination Of the above mentioned metals. Depending on the design and cost there may be roller, ball, bush or just no bearings at all.

[@fbi3865]

Why is the inlet bigger than the outlet?

[@Hydraulic.Online]

inlets on just about all pumps are bigger that outlets as it allows for better and more complete suction without any damaging cavitation.

[@MuhammadAfzal-jm6cw]

Hi

[@Hydraulic.Online]

Hi Muhammad, what's on your mind?

[@AndrewSpencer2]

The concept of seal direction was very poorly explained throughout the first half of this video.

[@Hydraulic.Online]

I dissagree Andrew, try watching again from 0:00 to 3:00 minutes. And tell me what you see and hear.