Selecting hydraulic motors and a pump for hydrostatic transmission

[AielloJ]

Where is the minimum RPM specified? The pressure compensator might be unhappy and unable to settle at very low RPM, but with zero layshaft RPM you do not require flow, because the vehicle is not moving.

On page 10 (using a PDF viewer), you should see a table for HPV6. Minimum RPM is 500. Yes, that is true for when the vehicle is stationary, but in the event, I need to travel slow, the intermediate shaft may not be rotating at the speed necessary to keep the pump's shaft rpm above 500 RPM. Granted, I'll be spending 90% of my time going above the threshold. http://www.zeushydratech.com/wp-content/uploads/2015/04/HPV_Series_Axial_Piston_Pumps_Form_260085_Rev_7-12.pdf

Where did you see that? Obviously, running a big pump slowly is inefficient and heavy.

I think it's because of the possibility of not enough lubrication for the shaft bearings. I'm very sure those parts not getting enough lubrication will guarantee a shorter lifespan.

How will you start and stop if you have no clutch?

Ah when I said no clutch, I was more thinking of a clutch that will engage and disengage the hydraulic pump to the rest of the gearbox. That was what I was heavily thinking of what I wrote that post. I still plan on attaching my CVT to the gearbox.

 

[Baluncore]

On page 10 (using a PDF viewer), you should see a table for HPV6. Minimum RPM is 500.

That is because the pump is usually driven by an electric motor or an IC engine, all of which have minimum speeds when operating above 500 RPM. Avoid using the HPV6 hydraulic pump. There must be something that "gives", a clutch or rear wheel spin, else the engine will stall when loaded at low RPM.

I think it's because of the possibility of not enough lubrication for the shaft bearings. I'm very sure those parts not getting enough lubrication will guarantee a shorter lifespan.

You are imagining it. Find a reference. Hydraulic pump bearings always run submerged in hydraulic oil, so shortage of lubrication is improbable.

Ah when I said no clutch, I was more thinking of a clutch that will engage and disengage the hydraulic pump to the rest of the gearbox. That was what I was heavily thinking of what I wrote that post. I still plan on attaching my CVT to the gearbox.

Then I ask the question; Where between the engine and the rear wheels is the clutch that enables the vehicle to be safely started and stopped?

 

[Tom.G]

Where is the minimum RPM specified?

Page 56 of the link given in post #51 shows:

OPERATING
Maximum rpm see below
SPEEDS
Rated rpm 1750
Minimum rpm 500
POWER INPUT @ 1750 rpm hp 15

Page 10 of the same document shows this footnote in the spec. table:

** Pumps operating at less than 150 psi (10.3 bar) may overheat and shorten
pump life.

What if I modified the system to use chain drive

Page 5 of the same document states:

DRIVE COUPLING
Jaw type with a flexible web is recommended. Tire
and chain type couplings are NOT recommended.

RTFM folks, it saves $$$.

Cheers,
Tom

 

[AielloJ]

That is because the pump is usually driven by an electric motor or an IC engine, all of which have minimum speeds when operating above 500 RPM. Avoid using the HPV6 hydraulic pump. There must be something that "gives", a clutch or rear wheel spin, else the engine will stall when loaded at low RPM.

The rear wheels should be able to turn at low RPMs. What's your reasoning for veering away from that pump?

You are imagining it. Find a reference. Hydraulic pump bearings always run submerged in hydraulic oil, so shortage of lubrication is improbable.

I can't find any, so that's why I said earlier I'll just go with it and find out myself.

Then I ask the question; Where between the engine and the rear wheels is the clutch that enables the vehicle to be safely started and stopped?

The CVT? I'm not sure what you're getting at here. Anyways, that is not my point of concern since this is a tangent off of pump RPMs.

 

[Baluncore]

The CVT? I'm not sure what you're getting at here. Anyways, that is not my point of concern since this is a tangent off of pump RPMs.

No. Not the CVT, unless it is also a clutch.
It appears we can no longer communicate.

 

[AielloJ]

If you'd like, we can start over to the question that restarted this, and I'll avoid going off tangents:

Are you aware of any significant detrimental side effects in terms of pump performance or life expectancy by having the shaft run below the minimum RPM "occasionally"?

 

[Baluncore]

Read the manufacturer's data sheet for the specifications. Then look at your application to evaluate what is relevant, and how by changing your application design you might work around those restrictions.

 

[Baluncore]

Most simple hydraulic pumps operate happily at zero outlet pressure. Pressure only appears when work is to be done and the fluid faces an obstruction to flow. A relief valve is used to prevent over-pressure. A pressure compensated pump may not require there be a relief valve in the system.

If there is a minimum pump pressure specified, then ask why that might be. Consider that at the minimum operating pressure the pump body will expand sufficiently to give normal operating clearances. A pump operated below the minimum pressure may overheat and wear due to insufficient clearance.

A pump having a specified minimum pressure may overheat at low RPM if it is unable to build up pressure in the circuit. The pump must have sufficient capacity to more than satisfy the total flow requirement of the hydraulic circuits at all operating speeds and in all modes of normal operation. If the hydraulically driven wheels lose traction and spin, on ice, oil, or in mud, the pump output pressure may fall below specification.

If you have a pressure compensated pump and you want to disable the hydraulic motor drive, you must do one or more of the following;
1. Stop the pump; for zero pressure and zero flow.
2. Close the pump outlet; which forces pressure to the set point, and flow to zero.
3. Bypass the motors; for maximum pump flow at zero pressure.
If you want freewheel you must include; 3. Bypass.
If there is a minimum pump pressure specified you must use; 1. Stop, or 2. Close.

If you can find a pressure compensated pump with a zero minimum pressure specification, preferably one you can turn by hand, then it should be easier to design and implement the system.

 

[wcwildcats12]

I have been looking into hydraulic driven “go carts” or vehicles for about 6 months now. I have read all of the post and comments from December of 2020. I’m curious to see if you have made any progress on this project. Pictures or what your decisions ended up being.