Selecting hydraulic motors and a pump for hydrostatic transmission

[Baluncore]

The pressure compensated pump will not overheat at low RPM. If the vehicle is moving the fluid will circulate. If the vehicle is stationary there will be little flow. Remember that the energy required is proportional to flow volume multiplied by pressure drop. Zero flow, or zero pressure, represent the lowest energy requirement.

The maximum possible pump flow at any time is proportional to RPM. When the pump is idle running at a few hundred RPM, with the vehicle and motors stationary, the flow will be very low, leakage only. The pressure should be regulated by the pump to the high set point, so the positive displacement motors will apply the preset torque to the wheels, but there will be very little energy flowing to the pump from the motor. That is because the pressure compensated pump will have reduced it's volume to almost zero, just enough to satisfy the flow of pump and motor clearance leakage.

 

[AielloJ]

So if that's the case, why do most axial pumps have a minimum RPM rating?

I would imagine that if I'm operating a pump in the regime of 0 RPM to the minimum working RPM, it won't produce enough flow to create the pressure necessary to provide torque to the front wheels, so the front two wheels won't be rotating, especially in the situation where I'm at rest, and I want to floor the gas pedal, the rear wheels would only be actively rotated until the pump passes the minimum rpm.

But you're saying that when the vehicle is moving at 5mph, or 200 RPM for the pump's shaft and the pump's minimum RPM is 500 RPM, the pump will keep the preset pressure in the lines within this regime?

 

[Baluncore]

So if that's the case, why do most axial pumps have a minimum RPM rating?

Please give me an example specification for a pressure compensated axial pump with a minimum RPM rating, so I can work out why there is a minimum RPM specified.

But you're saying that when the vehicle is moving at 5mph, or 200 RPM for the pump's shaft and the pump's minimum RPM is 500 RPM, the pump will keep the preset pressure in the lines?

No, I do not say that because I do not have a manufacturers data sheet for that pump you are talking about. Also I do not know what gear ratios you will have at 5 mph. You should engage a lower gear and so get a higher engine and therefore pump RPM.

What is the idle RPM of the engine drive to the pump? The rear wheel RPM is proportional to engine RPM, as is the pump RPM. You cannot drive the pump at close to zero RPM, that is well below the engine idle RPM. You must calculate the wheel RPM and the motor flow required. If the pump is able to supply sufficient flow, it will be able to regulate pressure.

 

[AielloJ]

Please give me an example specification for a pressure compensated axial pump with a minimum RPM rating, so I can work out why there is a minimum RPM specified.

Nowhere I can find a minimum pump RPM for this pump, but I talked to the manufacturer who said there was a minimum RPM. http://www.zeushydratech.com/product/vickers-pvb6-axial-piston-pump/

HPV6 is the next closest one which does clearly state it has a minimum RPM of 500.
http://www.zeushydratech.com/wp-content/uploads/2015/04/HPV_Series_Axial_Piston_Pumps_Form_260085_Rev_7-12.pdf

What is the idle RPM of the engine drive to the pump? The rear wheel RPM is proportional to engine RPM, as is the pump RPM.

The engine is connected to a CVT, and the engine's idle RPM is 1800 RPM, and max RPM of 3600.

You cannot drive the pump at close to zero RPM, that is well below the engine idle RPM. You must calculate the wheel RPM and the motor flow required. If the pump is able to supply sufficient flow, it will be able to regulate pressure.

I've already calculated this and determined the gearing. At max vehicle speed, I've determined the pump's max shaft speed should be 1700 RPM. There I've created a gearing ratio between my intermediate shaft (max RPM of 2030 RPM) that's driving the pump shaft. Then, using these ratios, I can say that at a vehicle speed of 12mph the pump's shaft will be at 500 RPM.

I can easily see myself going from 0 RPM to 500 RPM by flooring the gas pedal and needing torque from all four wheels, so the only part I'm confused about is how the pump performs when I'm increasing the pump's RPM from 0.

Edit:
I guess having a Pressure vs RPM graph could easily describe this. If the slope of the graph was almost instantaneous to the preset pressure value (in blue), then I can see this pump working since the front wheels will still be able to rotate at relatively low RPMs.

If it was an exponential growth through (in red), that would mean the pump would reach the preset pressure value too late, thus the two wheels wouldn't be rotating when I'm accelerating up to 11 mph in this regime. AKA, the front two-wheel motors will start rotating at 11 mph, when I want them to rotate with the car at like 5 mph.

 

[Baluncore]

At max vehicle speed, I've determined the pump's max shaft speed should be 1700 RPM. There I've created a gearing ratio between my intermediate shaft (max RPM of 2030 RPM) that's driving the pump shaft.

The engine is connected to a CVT, and the engine's idle RPM is 1800 RPM, and max RPM of 3600.

The engine RPM varies over a factor of two from idle to maximum RPM. If the pump will spin at a maximum of 1700 RPM, then it will idle at 850 RPM. I see no problem if the pump has a minimum rated speed of 500 RPM.

 

[AielloJ]

I should've mentioned there's a CVT in there, so the entire gearbox won't be rotating until the engine's rpm of the shaft reaches a minimum threshold, and even with the engine running at idle, the pump shaft will be at 0 RPM.

In the event, I want to accelerate, I still do think there is an issue with the minimum pump's rpm, since when I'm within the regime of 0 RPM and the minimum RPM, there might not be enough flow from the pumps shaft to create sufficient system pressure to allow the front two motors to rotate.

 

[Baluncore]

In the event, I want to accelerate, I still do think there is an issue with the minimum pump's rpm, since when I'm within the regime of 0 RPM and the minimum RPM, there might not be enough flow from the pumps shaft to create sufficient system pressure to allow the front two motors to rotate.

You are correct. Zero RPM is zero energy flow, so the motors will not deliver torque, or rotate.

You need to drive the pump in order to move the fluid, and to build up and maintain a set pressure. If the vehicle and motors are static, there can be no flow. At zero RPM the pump pressure compensator spring will push displacement out to the maximum, but still generate no flow. There is very little elastic volume change on the pressure side of the system, and the fluid is not compressible, so pressure should build up rapidly, and begin to regulate within the first turn of the pump.

 

[Tom.G]

AND then the question become: Does the vehicle start moving before the pump is effective? If so, the front-wheel motors will try to pump the hydraulic fluid, decreasing the acceleration. Hmm...

 

[Baluncore]

AND then the question become: Does the vehicle start moving before the pump is effective?

Yes.
The rear wheels will turn as the gearbox layshaft turns. The pump will turn at a proportional rate. The pump has capacity to produce a greater volume than is required by the hydraulic motors at any road speed. So the front wheel motors must turn and pressure must build up in the compensator as it matches the pump flow rate to what is required by the motors.

We are in the dark here since the vehicle configuration has not been defined. Does the vehicle have a manual or centrifugal clutch before the gearbox and layshaft/pump? Where in the system is the CVT that appeared in post #39?

 

[AielloJ]

Here's a schematic of my idea. No clutch yet, I want to avoid that if possible. The input shaft is connected to the CVT, which is connected to the engine. So if I'm understanding this correctly, within a certain vehicle speed (say 0 mph, and 10mph, 10mph is arbitrarily picked in this case where that's the minimum vehicle velocity before pump shaft begins to rotate at the minimum RPM), the front two motors will work against the vehicle's acceleration, but the pressure should begin to build up rapidly within 0mph, and 10mph?

 

[Baluncore]

It depends on whether your rear wheels spin during the start, and the design of the pressure compensator feedback control system.
Why not drive the hydraulic pump with a belt from the front end of the motor?

 

[AielloJ]

Hm motor? You mean the engine shaft?

Edit:
What if I modified the system to use chain drive, so that the gearing between the engine shaft and pump shaft is almost 1:1 so that the pump is always spinning with the engine idle? But that'll cause more issues if I'm stationary, the pump will be trying to run the motors but my brakes will prevent that

 

[Baluncore]

What if I modified the system to use chain drive, so that the gearing between the engine shaft and pump shaft is almost 1:1 so that the pump is always spinning with the engine idle?

The engine speed is 1800 – 3600, the pump maximum is 1700, so you need a drive ratio of 1700 / 3600 = 0.47;
It can be done with a V-belt that will absorb shock and slip if needed. Roller chain is a more expensive solution. The pump may need a shaft support bearing to counter the side force of the drive belt.

But that'll cause more issues if I'm stationary, the fluid will want to go somewhere.

If the hydraulic motors are stationary then the pressure will build up torque on the front wheels, but they will not rotate since the pump will reduce flow volume to cover internal system leakage only. (Zero flow) * (max pressure) = (low energy), so there would be little load on the motor engine. A bypass valve across the motors could be used to zero the motor pressure torque and operate at; (maximum flow) * (zero pressure) = (low energy). That would provide freewheel while idling.

 

[AielloJ]

I guess it sounds like I'll need to find a workaround for the pump's minimum RPM right?

It appears that running the pump at lower RPMs than recommended will quickly shorten the pump's lifespan.

Edit: I'll do a 180 on that. I can't find any literature about this, so I'll figure it out myself and implement it on the gearbox anyway, since I'm expecting to run the pump within it's normal working conditions 90% of the time.

 

[Baluncore]

I guess it sounds like I'll need to find a workaround for the pump's minimum RPM right?

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.

It appears that running the pump at lower RPMs than recommended will quickly shorten the pump's lifespan.

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

so I'll figure it out myself and implement it on the gearbox anyways

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

 

[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.