Piston motor speeds

[abasinger]

Greetings,

I would like to see the thoughts of others on maximum piston motor RPMs. I have a small fixed displacement axial piston motor I would like to use for a hydraulic chainsaw. What maximum RPM would anyone estimate before reliability becomes significantly risky? The motor is 5CC in displacement/rev and rated for 4000 psi.

For intermittent operation that the chainsaw would involve, does anyone percieve that 8-10k rpm is in general a lot for an axial piston motor of this size to handle? The total diameter of the housing is around 3.5", so I cant imagine the rotating group is very large at all, which would help during high rpms. Im also not an engineer in any professional sense.

Thanks in advance for input!

 

[FluidPowerUser]

The speed limit is set by the shaft seal and bearings. High speeds will limit the bearing and seal life.

 

[Stick.]

Looking at the Parker catalog (just to get a sense of things), out of their 57 options for hydraulic motors, only 3 are rated for speeds in that neighborhood, so in general, speeds that high don't seem common. What does the manufacturer of your motor list as the rated max speed?

 

[3DDave]

That's nearly 700 cc's per second of fluid flow at 8000 rpm. I would expect large fluid friction losses in trying to push fluid that fast into a tiny motor.

 

[FluidPowerUser]

A pump pushing oil through a motor will continue to do so until the pressure at the inlet of the motor goes above the max pressure. The motor will run at a speed that is proportional to input flow and the displacement.

A pump of the same displacement would be speed limited by it ability to pull fluid in without vapourising the oil in the inlet gallery. A motor does not have that limitation as the pump pressure will always be high and will get higher with speed.

The shaft bearings are often plain bearings and they have a limited speed rating before the tribology breaks down. The same with shaft seal, the velocity of the shaft rotating against the seal lip will get hot and it will wear very quickly at high speeds.

Higher speed motors are available, they have ball or roller bearings to support the shaft and the shaft seal material is more resistant to wear.

The small motor that is on the chainsaw will start to leak from the shaft as the earliest indication of speed related issues. It may start to make noises as the extremely fast fluid creates high velocity jets, but there won't be any cavitation as the pump pressure will be close the max working pressure as a function of the pressure drop across the motor.

The Parker catalogue is for bent axis and gerotor type motors. The motor in use here is a small axial piston motor. There is a difference to the way these motors respond to high flow input.

 

[abasinger]

Good information, thanks to all.

I was able to dig up one more bit of technical info- the motor is rated for 5000 rpm. 5k rpm would still be workable with a larger sprocket/bar combination.

For what it's worth, i found that the source supplier of the motor is rexroth/Bosch. Part number/model is R902495615. Couldn't find any catalogs on it. Not sure if anyone has access to more information?

 

[FluidPowerUser]

Are you confident on that number? The minimum displacement on those A10 motors is 10.6 cc/rev.

If this is the motor that is fitted, then the max speed is dictated by the mechanics of the swash plate and the reciprocal motion of the piston.

The statement on the data sheet is pretty clear:

▶ Operation above the maximum values or below the minimum values may result in a loss of function, a reduced service life or in the destruction of the axial piston unit. Bosch Rexroth recommends checking loads through tests or calculation/simulation and comparing them with the permissible values.

Adding a larger sprocket won't change the speed of the motor, it will only change the speed of the chain and increase the inertia of the rotating assembly. It also changes the forces acting on the motor. The motor has a specific power capacity based on displacement and pressure or torque and speed, with speed a derivative of displacement.

When you stop accelerating the motor, it will turn in to a pump as the inertia of the chain will try to drag the shaft. If you exceed the fill limits of the motor by running it above 5000 RPM, it will pull the slippers off swash plate and make a very loud and angry noise.

 

[abasinger]

Are you confident on that number? The minimum displacement on those A10 motors is 10.6 cc/rev.

If this is the motor that is fitted, then the max speed is dictated by the mechanics of the swash plate and the reciprocal motion of the piston.

The statement on the data sheet is pretty clear:

▶ Operation above the maximum values or below the minimum values may result in a loss of function, a reduced service life or in the destruction of the axial piston unit. Bosch Rexroth recommends checking loads through tests or calculation/simulation and comparing them with the permissible values.

Adding a larger sprocket won't change the speed of the motor, it will only change the speed of the chain and increase the inertia of the rotating assembly. It also changes the forces acting on the motor. The motor has a specific power capacity based on displacement and pressure or torque and speed, with speed a derivative of displacement.

When you stop accelerating the motor, it will turn in to a pump as the inertia of the chain will try to drag the shaft. If you exceed the fill limits of the motor by running it above 5000 RPM, it will pull the slippers off swash plate and make a very loud and angry noise.

Confident on the pressure displacement and rpm figures- they came from a print sheet of the motor in its rebranded form. I suppose errors do occur but it's an official document. The R number was also provided with those specifications.

Where were you able to find out that it is an A10 series motor? I tried a couple times getting ahold of a rexroth rep but haven't had a return call yet.

Right, I hadn't mean to suggest that the sprocket would change rpm but the chain speed is the target trying to hit. 5000rpm on a 15 tooth sprocket gives a mid range chain speed. This is just for home use, not aiming for any record breaking sawing action...just want to stop cutting with a gas driven.

I am somewhat certain that this particular motor has a check valve between work ports built in? It's on the schematic as an internal part, but I could be mistaken. At a minimum, if a check between work ports was added externally or even along with a makeup check supplied by a tank source, this should alleviate any issues when stopping the motor? After doing some more figuring, I think I'm happy with staying below 5000 rpm and having a low chain speed if it means this motor will be usable.

If the rpm figure is correct flow should be limited to around 6.5 gpm or less...and I only have 3300 psi available so that is self fulfilling.

Some practical numbers compared to a large gas saw. Stihls largest chainsaw model MS881 is capable of ~ 7 ft lbs of peak torque at around 7500 rpm= 10 HP. A 5cc hydraulic motor should produce 12 ft lbs at 3000 psi....and if it runs at 5000 rpm- ~ 11.5 HP.

The MS881 typically can drive a 7 tooth sprocket- .404" pitch at 7500 rpm= 21,210 inch/min or 29 feet per second

The MS881 at 11,000 rpm on 7 tooth sprocket= 43 feet per second.

Since we have options and a mill and lathe, I plan on adapting a suitable 15 tooth sprocket to the hydraulic motor(and suitable width bar to match) - .404" pitch at 5000 rpm = 30,300 inch/min or 42 feet per second.

As far as force realized at the chain due to different sprocket sizes, I thought it to be helpful to express the relationship between the gas saw and hydraulic motor performance as torque/sprocket teeth since the diameter of the sprockets increases directly with tooth count, given the same chain pitch.

MS881- 7/7= 1.0
Hydraulic Frankenstein= 12/15= 0.8

My spaghetti throwing is almost over- the increased sprocket size will mean that the hydraulic version will have 20% reduction in performance vs the largest gas saw that Stihl produces. It seems like this should work at least to some satisfactory capacity? Are my hydraulic numbers somewhat correct? I don't expect any verification on the saw power figures.

I currently use a gas saw less than half the size of the example above, just for reference. I don't desire forestry harvester performance, and if it was equal to my much smaller gas model without having to manually use the saw and maintain an engine, it would be joy. Also for reference, the power source is a skid steer hydraulic system.

This is good discussion! I appreciate the talking points very much!

 

[FluidPowerUser]

I used to work at Bosch Rexroth and now I work at a distributor for Bosch Rexroth. Try the link below.

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www.boschrexroth.com

You had said 5cc/rev for the motor displacement, I was just checking.

Yes, the motor could have an anti cavitation check valve on the inlet/out ports. Need to confirm the part number to be sure. You can add something externally to help the motor stay full on run down. Adding the check valve will allow the motor to slow down steadily, in a couple seconds most likely.

Rated power of the 10.6cc motor is 24.7kW or 33HP. This is at max flow of 53 LPM (14 GPM) and delta P across the motor of 280 BAR (4100 PSI). If you only have 227 BAR (3300 PSI) you torque will be lower and so will the power. You'll get 38.2 Nm (28.17 ft lbs) against the peak torque of 47 Nm (34.6 ft lbs). Power will come down to 20.051 kW (26.8 HP).

It will work, but you will be power limited verses what the motor is capable of. It's the power available from the skid steer that is limiting you.

You can make the sprocket as big as you want and that will speed the chain up, but the torque will have to reduce as the power will be constant. Power = torque x speed. The rate of cut will have to slowed to compensate for velocity of the teeth on the chain.

 

[abasinger]

I used to work at Bosch Rexroth and now I work at a distributor for Bosch Rexroth. Try the link below.

Search

Search

www.boschrexroth.com

You had said 5cc/rev for the motor displacement, I was just checking.

Yes, the motor could have an anti cavitation check valve on the inlet/out ports. Need to confirm the part number to be sure. You can add something externally to help the motor stay full on run down. Adding the check valve will allow the motor to slow down steadily, in a couple seconds most likely.

Rated power of the 10.6cc motor is 24.7kW or 33HP. This is at max flow of 53 LPM (14 GPM) and delta P across the motor of 280 BAR (4100 PSI). If you only have 227 BAR (3300 PSI) you torque will be lower and so will the power. You'll get 38.2 Nm (28.17 ft lbs) against the peak torque of 47 Nm (34.6 ft lbs). Power will come down to 20.051 kW (26.8 HP).

It will work, but you will be power limited verses what the motor is capable of. It's the power available from the skid steer that is limiting you.

You can make the sprocket as big as you want and that will speed the chain up, but the torque will have to reduce as the power will be constant. Power = torque x speed. The rate of cut will have to slowed to compensate for velocity of the teeth on the chain.

Very interesting.

Caterpillar part number is a 333-7828, would you be able to find any information on that through your BR connections? It looks similar to the ones on the page in your link- but has STOR ports for A and B (case drain too) Maybe it was custom built to 5cc for caterpillar? You have me curious now, any information available on the internals? Is the swashplate fixed by physical shape of it or is it on shims or an internal adjustment?


Edit- I see that I didnt look far enough down. On page 19 looks like it. Found a basic repair manual too that shows what looks like a swashplate that is fixed by how it's machined?

 

[abasinger]

Very interesting.

Caterpillar part number is a 333-7828, would you be able to find any information on that through your BR connections? It looks similar to the ones on the page in your link- but has STOR ports for A and B (case drain too) Maybe it was custom built to 5cc for caterpillar? You have me curious now, any information available on the internals? Is the swashplate fixed by physical shape of it or is it on shims or an internal adjustment?


Edit- I see that I didnt look far enough down. On page 19 looks like it.

 

[FluidPowerUser]

I was the hydraulic engineering manager at Caterpillar before moving to BR. Yes, they could very well have changed the swash angle to limit the displacement.

Internals of the A10 motor are like this.





Swash plate is fixed at an angle to give the relevant displacement. Piston diameters are increased to give more displacement.

You can't adjust the swash angle. On variable motors, there is a big piston and a yoke that swivels the swash plate to vary the displacement.

When you look at the image, you can't see how the speed can affect the reciprocal movement of the pistons. There is a limit the amount of momentum that can be handled inside the rotating group.

 

[abasinger]

Got it. I hadnt thought that they just change the piston size. You read my thoughts! Cool that you worked for Cat!

So if its indeed 5cc I would only be getting 1/2 your 26.8 hp figure right?

 

[FluidPowerUser]

Yes, if the displacement is limited to 50% of the standard motor, the power and the torque will be reduced by the same amount.

 

[abasinger]

Yes, if the displacement is limited to 50% of the standard motor, the power and the torque will be reduced by the same amount.

Many many thanks!!