Hydraulic motors in series

[WezTec]

Hi Guys

Little help with series connected hydraulic motors. Specs are just examples.

If I needed 100 Bar with a torque of 50Nm in parallel to drive, would I need 200 Bar for two motors in series? Intern this would increase the torque 100Nm.? would the torque be halved per motor, 50Nm per motor ?

Thanks

 

[MikeHalloran]

There's a fairly large conversation about this very subject, and not terribly long ago. It details a number of problems you may/will have to solve.

What it boils down to is that you are generally better off with one motor of the proper size for your job. ... and you have not provided nearly enough information to allow anyone to evaluate why you might need more than one motor.





Mike Halloran
Pembroke Pines, FL, USA

 

[WezTec]

Okay sorry didn't think I should bore you with the details as I am sure there must have been discussions of this nature. I did search for series motors but only 4 lines came up which helped me nothing.

I received and enquiry asking to use hydraulic motors on a new 4x4 agriculture machine.

Customer requires two speed operation, low speed for loading and high speed for long distance travel.

Details:

2 x ZF 45% limited slip diff’s coupled to Eaton bent axis motors, one per diff’s. Motor can take full back pressure with limitations between motor 1 & 2 which is not the problem

Eaton hydrostatic pump with manual linkage

When it’s in parallel it has high torque at “200Bar” when in series I will have a problem because I only have 200Bar to deal with, what I am trying to work out is the gradient it will climb with a lower torque, what happens to the torque with 2 motors in series ?

 

[Oldhydroman]

Disregarding any complications with slightly unequal flows when in series, and different working efficiencies because of the different outlet pressures, the simple answer to your question is that when you run the motors in series you halve the torque output of each motor.

Think of the pump output as being the hydraulic power you have available to you. There is a maximum pressure of 200 bar and let's say, just as an example, that you have a maximum flow of 300 L/min. That comes to a hydraulic power of 100 kW. Your motors will convert this hydraulic power (flow x pressure) into mechanical power (torque x speed). If you fit a HUGE motor you will get lots of torque output from the available pressure but not many rpm from the available flow - in this case the output will be: big torque x small speed = 100 kW minus losses.

Conversely, if you fit a tiny motor you won't get much torque but you will get a tremendous shaft speed - in this case the output will be : small torque x big speed = 100 kW minus losses. In all cases the mechanical power output of the motor will be equal to the hydraulic power input to the motor multiplied by the overall efficiency of the motor (the efficiency is always less than 100%).

When you put two motors in parallel they will share the flow between them (say 150 L/min each) but will each have the same pressure available to them (200 bar). On your machine this will give low speed, high torque operation. When you connect the motors in series they will each have the full pump flow going through them (300 L/min ... even though the downstream motor gets the flow secondhand) but the motors will have to share the pressure (100 bar each in ideal circumstances but it might be an unequal share, say 120 bar and 80 bar). This mode of operation will give you high speed, low torque operation. The analogy is one of changing to a higher gear in a standard automotive transmission. Compared to parallel running, each motor will run at double the speed but only half the torque. The motor power outputs remain the same. (A more comprehensive analysis of the efficiencies will modify this answer a little.)

I do hope you're not trying to engineer a hydrostatic transmission yourself because the intricacies of the component selection process and the complications of the circuit design predict a very steep learning curve for you. There are, however, some pre-designed solutions which you might want to avail yourself of. I'm thinking of the Rexroth pumps and motors which have an automotive style control ("DA" control). This scheme is like a continuously variable automatic transmission which starts off in "low gear" and gradually increases the gear ratio according to engine speed and load on the motor (machine weight, acceleration, steering, gradient, rolling resistance etc.). Sauer-Danfoss offer something similar. Alternately you might want to look at using a pair of variable displacement hydraulic motors which use a hydraulic pilot signal to reduce their displacements (to give you the high speed/low torque mode). It is possible to incorporate a high pressure override in such a motor displacement controller which causes the motor displacement to increase (regardless of the command signal) if the gradient encountered during high speed running becomes too much for that mode of operation. The advantage of a hydraulic pilot signal is that you can use the same signal to control the pump displacement; If you had, for example, a joystick with an output of 0-20 bar, you could set up the controls so that 2-10 bar joystick output pushes the pump displacement from 0% to 100% and the remaining 10-20 bar output pushes the motor displacement from 100% to 50%.

Either way, without wishing to be rude, if you have such small understanding of the behaviour of hydraulic motors running in series or parallel, you are nowhere near well enough equipped with all the knowledge you will need to create a workable, reliable, safe, efficient and properly coordinated hydraulic system for a vehicle drive. I'm presuming this will be a man-riding machine and hence potentially laden with all sorts of legislative processes in the event of there being any injury which could be remotely attributed to your design. The fact that you've even asked this simple question on Eng-Tips could be construed, in some future court case, as "proof" that YOU shouldn't be designing such a system. Please tell me that there is someone suitably qualified overseeing your designs. If not, then do yourself a favour and involve someone who is qualified, and involve them in such a way that they carry some professional responsibility (you're going to have to pay them).

DOL

 

[WezTec]

Let me start off by saying that my wording was not worded correctly, I have been “assigned” a task to come up with a system for this machine which of course will have to be approved by my Engineer that been said I did not know that this site was just for Engineers!!

But before I go I just like to clarify that I have done “only 2” hydrostatic systems in the past, yes my knowledge is limited on hydrostatic drives but am intrigued to learn more about it.

Both of them were two motors in parallel. I know of the different controls, remote pilots, joysticks, purging, efficiencies but the difference with this assignment is that they want 2 speeds and must be simple. Hence been bent on the idea of manual linkage, but your suggestion with remote pilot I would say is still classed as “simple”.

I found a two speed circuit in a SAI catalogue which leads to my question about the torque in series motors. I knew you can drive them in series and had an idea of what the answer might be but just needed clarification.

“The advantage of a hydraulic pilot signal is that you can use the same signal to control the pump displacement; If you had, for example, a joystick with an output of 0-20 bar, you could set up the controls so that 2-10 bar joystick output pushes the pump displacement from 0% to 100% and the remaining 10-20 bar output pushes the motor displacement from 100% to 50%”

If it’s not too much trouble to answer, with regards to the last section would I be right in saying that when the joystick is pushed forward half way you will have max speed low torque and full forward max torque low speed ?

Thanks for your input

 

[Oldhydroman]

Hi WezTec

It's no trouble at all to answer your question - but it's a long answer.

First of all, fix into your mind the fact that the majority of closed circuit pump displacement controls start at 0% displacement and the application of some form of control signal pushes the displacement towards 100% (actually +100% for one direction of flow and -100% for the other direction). Then add to that thought the idea that the motor displacement control I'm about to describe to you is the opposite way round to the pump displacement control: the motor starts off at 100% displacement and this is pushed towards a smaller value by the application of the control signal.

If you go for the single joystick approach I mentioned previously (it was just an idea to whet your appetite) then it will only be the first portion of the joystick travel which will push the pump to full displacement. Just at the point where the pump displacement reaches a maximum the system will be running at full flow with your motors also at full displacement. That will give you maximum possible torque output and a certain speed. Let's call this speed 10 mph just so we can give it a number.

The remaining part of the joystick travel just controls the motor displacement (because the pump displacement control is now complete). Once the pump is on maximum displacement there will be no more flow going into the system so the maximum possible power input to the system will now be fixed. Reducing the motor displacement allows the same flow to produce more speed but it is at the expense of a reduction in maximum torque output. Think of the motor displacement control as a form of "overdrive".

You wouldn't want to reduce the motor displacement all the way to zero because that would create a device which would attempt to convert your hydraulic power input (flow x pressure) into a mechanical output of infinite speed and zero torque (actually the motor wouldn't drive - it would just freewheel). So you need to pick a value for the minimum motor displacement and adjust the motor's minimum displacement stop screw accordingly. If you set the motor so that its minimum displacement were 50% then, when you push the joystick lever fully over, the pump would still be on full displacement but you would now get a speed of 20 mph (only half as much maximum torque though). If you set the motor minimum displacement to 33% then full actuation of the joystick would give you 30 mph (but only 33% maximum torque).

By way of a worked example consider a system using a Rexroth A4VSG pump with the HD1 displacement control (the pump displacement control characteristic is: 6 bar = 0%, 18 bar = 100%, <6 bar = pump remains at 0%, >18 bar = pump remains at 100%, maximum permissible pressure signal = ? you need to confirm this with the manufacturer but likely to be at least twice the maximum required command pressure signal).

Now couple this pump to any number of Rexroth A6VM motors (connected in parallel) also with HD1 control (motor displacement control characteristic is: 10 bar range from 100% to 0% displacement with "start of control" settable from 6 bar to 22 bar). Set the "start of control" to 19 bar and set the maximum displacement stop screw to 50% displacement. You will now need a pilot signal to the motor displacement control of between 19 bar and 24 bar: 19 bar = 100% displacement, 24 bar = 50% displacement, <19 bar = motor still at 100% displacement, >24 bar = motor stays at 50% displacement. [Note that if the motor displacement isn't going all the way to zero then you don't need to supply the full 10 bar range on the pilot pressure signal.]

To control the scheme use a Parker PCL402 joystick with a number 7 spring (output = 5 to 24.9 bar). You have to push the joystick to about 26% travel before the pump first comes off 0% displacement - this "deadband" in the output of the joystick about its centre position is deliberate because you don't want the machine movement to be on a "hair trigger". So at 26% joystick actuation the machine will begin to move. As you push the joystick handle further and further the machine speed increases. At about 73% travel the joystick output will have risen to 18 bar and the pump will have reached full displacement. [The pump will now stay on full displacement even though the joystick can move further.] At about 77% travel the joystick output will be 19 bar and the motors will just about be starting to reduce their displacement. When the motor displacement reduces the shaft speed will increase (you will get more rpm for each L/min) but the maximum available motor torque will decrease (you will get fewer Nm for each bar). At about 97% travel the joystick output will be 24 bar and the motors will be running at 50% displacement. Your machine will have reached maximum speed - provided of course that the current maximum torque output of the motors is sufficient to drive the machine at that speed. The remaining 3% travel on the joystick does nothing - but it's nice to have it there to account for manufacturing tolerances.

These numbers are just an example using off-the-shelf components. There are a lot more joystick control characteristics than the few shown in the Parker datasheet. You can even get the joystick custom built to your exact requirements ... but the reality of the situation is that you are unlikely to find yourself asking for something that has never been done before. You won't actually need a "custom" build - you'll just need the manufacturer to dig out something that has a characteristic which will suffice.

If your boost pressure is at least 40 bar then you can use that as the pressure source for the joystick (you need a supply pressure ~15 bar higher than the maximum output pressure). It is likely that your boost pressure won't be this high so you could pick up a source from your auxiliary hydraulics (if there are any) or you could put a shuttle valve across the two outlets of the pump and use the resolved signal as the joystick source (you will almost certainly need a reducing valve as well because the joystick maximum supply pressure will be less than the maximum system pressure). You might find the pump already has this shuttle valve built in and you can access the pressure at this point from some auxiliary port on the pump itself.

Connect the tank port of the joystick to a spare port on the pump case. The joystick output pressure is actually 5-24.9 bar ABOVE the pressure in the joystick tank port and the pump displacement control requirements are actually 6-18 bar ABOVE the pump case drain pressure. Make both of these tank and case drain pressures exactly the same and there will be no nasty surprises.

You will also need a shuttle valve between the two joystick output ports. The output of this shuttle valve will be used for the signal to the motor displacement control. The pump has to go both sides of centre (drive forward or drive in reverse) but the motors only need to receive a copy of either pump control signal so that they can reduce their displacement accordingly. Unless, of course, you want to prohibit maximum speed running in the reverse direction. In fact, if you wanted the reverse drive direction to have a specific speed limit imposed on it you could choose one side of the joystick to have a different spring to the other; a number 4 spring, for example, will only output 15.6 bar at full joystick actuation - that's only 80% pump displacement and no reduction of motor displacement. Or you could wind in the pump maximum displacement stop screw for that direction of flow (this loses resolution on the joystick) or you could fit a pressure reducing valve to that particular control port on the pump and make the maximum reverse speed fully adjustable (still loses joystick resolution). [It is often the case that the maximum displacement stop screw can only reduce the maximum achievable displacement to 50% or sometimes only down to the displacement of the previous pump in the size range.]

You might want to offer a high pressure override on the motor displacement control. This is an option on the motor which will push the motor displacement back to a larger value if you try to work the machine too hard when in the high speed mode. It's just like your car's automatic transmission selecting a lower gear when you encounter a very steep hill. But be careful with this one because a reduction of joystick position will cause the machine to decelerate - the deceleration force comes from a high pressure in the outlet port of the motor. This deceleration pressure can be high enough to force the motor to a higher displacement which will create an even greater deceleration and a very rough ride ensues. To overcome these pressure selection problems you might want to look at the Rexroth DA controllers (both on the pump and on the motors).

You can consider applying some sort of safety logic to the displacement control circuits such as a weight activated limit switch on the operator's seat which operated a solenoid valve in series with the hydraulic supply to the joystick. Then if the operator ever fell out of the seat the machine would stop (this sort of scheme is common on fork lift trucks). Maybe there's an arm rest that has to be lowered to activate the travel drive - the same arm rest that the operator has to lift up to get out of the cab.

You could introduce a pressure reducing valve that was switched out of circuit by a solenoid valve to give an "inch" control. When the solenoid valve is de-energised the joystick only gets a very low pressure supply and the machine can only move around slowly (because you can never get much displacement on the pump). When the solenoid valve is energised the joystick receives the proper supply pressure and full machine functionality is resumed. This function is useful for [enforced] slow speed manoeuvering in tight spaces.

I suppose the trick is to think of all the scenarios that could usefully employ some sort of intervention in the hydraulic circuits in order to enhance the safety of the machine and/or reduce the risk to the operator. Offer these options to your customer with an description of the scenario you have envisaged and an explanation of how you propose to close out that risk (along with the price for incorporating that feature). Your customer might have said that they want a "simple" control (robust, reliable, easy to troubleshoot/repair etc.) but they will probably never say "Don't waste money making it safe". So explain the ways in which the machine could pose a danger to the operator (and/or others) and, if your customer doesn't want to spend money on that particular extra feature which mitigates that particular danger, then get THEM to tell you that they don't want it. Get/keep a record of the customer's rejection of the safety features you offered and hope you never have to show it to anyone from the legal profession (but sleep easier knowing that you have it filed away somewhere).

Please don't get me wrong, I'm no coward! I just don't believe in letting a bunch of people walk all over me at any time in the future merely because I tried to do someone else a favour by agreeing to produce a "simple" hydraulic circuit for the travel drive on a man-riding machine.

I'm pleased that you find the subject intriguing (it's been my whole career and I'm still intrigued as well), but I would advise you, my friend, to be extremely careful with both the design and the documentation.

DOL

 

[WezTec]

Hi

My main hydraulic experiences are more in Industrial than mobile and appreciate these comments.

I can only read in catalogues to get information but they don’t tell you all of what can go wrong as you have explain in very good detail. My first thought was series motors but after drawing out the circuit and amount of valves you would need just to change it to parallel, I must admit wasn’t a brain storming idea and since your first mail I have been thinking of other ideas.

Our company is obligated in using “Eaton” products which is one of my problems and the pumps we have are manual operated but I have found a remote controlled pump “Eaton HPV with a CA control”

Your suggestion on “inching” is most likely what they are requiring, thanks for that!

One of my thoughts was using a torque limiting pump with fixed motors your thoughts would be appreciated? Theory: Once the pressure rises to the gradient it will de-stroke the pump proportionally, this would be similar to the motor high pressure cut off but without the high pressure deceleration force facture? The “Eaton HPV with CA controller has this function”

“But be careful with this one because a reduction of joystick position will cause the machine to decelerate - the deceleration force comes from a high pressure in the outlet port of the motor. This deceleration pressure can be high enough to force the motor to a higher displacement which will create an even greater deceleration and a very rough ride ensues”

Please could you go into little more detail on this section not too sure I understand this? what I gather is when going up the hill the high pressure cut out will increase the motor displacement which will give you more torque and lower speed, if he shits the remote back whilst on the hill, the high pressure on the outlet port can force the motor back to a decrease in displacement but your pump will have a decrease in flow?

Thank you once again for your sound and very detailed advice. I really appreciate all your efforts in helping me understand the legalities and problems that can occur I will take this under serious consideration.

Wes

 

[WezTec]

"One of my thoughts was using a torque limiting pump with fixed motors your thoughts would be appreciated? Theory: Once the pressure rises to the gradient it will de-stroke the pump proportionally, this would be similar to the motor high pressure cut off but without the high pressure deceleration force facture? The “Eaton HPV with CA controller has this function”

Sorry don't answer this. We don't have a Kw problem, was trying to figure out the high pressure variable motor force you were explaining and found this control in our catalogue. Has a similar concept but no overdrive mode.

 

[Oldhydroman]

Sorry Wes - I've been away and couldn't respond to your last question.

The HPV pump is from Linde and there's some tie up between them and Eaton, don't know the in's and out's of it, somehow find marketing bulletins so boring that I just ... d.r.i.f.t.....a.w.a.y......yawn!

So have a look at the Linde HPV-02 pump datasheet (attached) and at the bottom of page 21 is a sample circuit showing the CA controlled pump with the HMV-02 EH1P motor and a couple of the other valves you need. The HMV-02 motor datasheet is a little tricky to find because there's a mistake on the Linde website and the link to give you the motor datasheet gives you the open circuit pump datasheet instead. I'll add another post with the motor datasheet attached (can't figure out how to attach two things - I'm getting old!)

I wouldn't worry about the odd valve or two having to come from a non-Eaton source - even in the Linde catalogue they tell you they don't make all the valves you need. Your employer obviously wants you to maximise the sales of the Eaton product, but if it takes longer (and hence costs more) to cobble together a system using the wrong valves then who have you helped?

The advantage of the CA control on the Linde HPV-02 pump is that it can be configured to match the torque/speed characteristics of your engine:
1) As you increase engine speed the pump displacement automatically increases and, when the moment is right, the motor displacement then decreases. The drive behaves like an automatic transmission in a car.
2)The inch control keeps the pump displacement low and the motor displacement high - so you only go slow regardless of engine speed.
3) The brake control forces the pump to a lesser stroke so that you get engine braking - further depression of the brake pedal usually applies the friction brakes as well.
4) There will be some form of high pressure override in the pump and a similar scheme in the motor (these have to be co-ordinated)

The niceties of the pump and motor displacement controls are usually too intricate for them to be easily represented using the fluid power symbols shown in the catalogue. So phone up Linde and have a nice long chat with someone in their technical dept. You might even get them to set the pump up to match your engine (there aren't that many engines around that you'll be using one they haven't come across before).

As for your other question about the pressure override selection in the motor, this is well explained in the HMV-02 motor datasheet (page 19).

You will need some solenoid controls in your drive system - otherwise how will the pump know to drive the machine forwards or backwards when you rev up the engine? There are two solenoids on the motor, one forces it to stay at high displacement (you could call this "low range") and the other is slaved across to one of the pump direction control solenoids for the purposes of selecting the right pressure signal.

Let us know how you get on.

DOL

 

[Oldhydroman]

Here's the motor datasheet.

DOL