Control option of Variable Displacement pump

[Uncia_Uncia]

Hello All,
I have question regarding the control option of Variable displacement pump.
I was going through variable displacement pump technical configuration. It was mentioned that load sensing controlling option is better
than pressure compensated control. I realize that load sensing covers the pressure compensated control, but I would like to know why
load sensing is better option. In which case I really need load sensing control, and in which cases I can get away with only pressure
compensated control?

 

[hydtools]

The choice depends on what you are trying to do.

Ted

 

[Uncia_Uncia]

Hello Ted,
Thank you for replying.Lets say I want to lift a load with telescopic handler or forklift. what option would be good and why?

 

[hydtools]

I would chose load sensing in order to maintain fixed flowrate with variable loads.
You could do the same with a fixed displacement pump delivering your required flowrate.

Pressure compensation control attempts to maintain constant pressure by varying flowrate.

Ted

 

[HPost]

If you go with a load sensing pump, then you will need to use a valve that has load sensing functionality too.

LS pumps work best when the speed of the input is varied, otherwise you would be best to use a torque limiting controller.

 

[hydtools]

HPost, with the load sensing control adjust the pressure compensator to the max pressure desired. That will limit the torque demand of the pump.

Ted

 

[HPost]

I disagree Ted, pressure compensators send the pump to minimum at the set pressure. Torque controllers adjust the pump displacement in proportion to the outlet pressure.

 

[hydtools]

Well, that is why I asked Roshan Teli what he wants to do. Constant flow? The what-if output device is a cylinder. Then load sensing, but not torque control. Torque control will vary flowrate, yes? I did not want to complicate the choices without more response from OP. Why consider a variable pump at all? What are the prime mover limitations? What work output is expected?

So far all shots in the dark.

Ted

 

[Uncia_Uncia]

Hello All,
Thank you very much for responses. I was studying my material to respond. Well, the prime mover is transmission with pump drive drive ratio of 0.971:1 and torque capacity of 245 Nm. The transmission maximum input RPM is 3300, but engine rated speed is 2200 RPM.
As hydtools mentioned output device is a cylinder which has to lift 6000 lbs with telehandler.

 

[HPost]

Get a pump with an LS compensator and a torque limiter. That will give you the energy saving capability and it will allow you to set the maximum torque at the pump to match the engine output.

LS on its own will allow you to match the demand at the cylinder, but if that exceeds the engine torque limit the engine will lug down.

When you purchase the pump, tell the supplier that you want 245Nm and they will set the compensator to that value. Then, if ever your load goes high while the pump is at a displacement that exceeds the torque, the controller will back the displacement off automatically.

Under that torque curve, the LS compensator will match the pump flow to the speed requirement.

You will need a valve that has an LS output that you can feed back to the pump to control it.

 

[hydtools]

Is the driving engine running with a fixed throttle position? Make sure the engine speed is set above the engine peak torque so that with load the engine speed droops towards engine increasing torque.

Are you powering more devices than the one hydraulic cylinder? If not, adjust the pump displacement stop to deliver the flow you need for the cylinder and you will not need the load sensing control.

HPost, will not the two controls give conflicting commands? Load sensing trying to increase displacement to maintain flow while torque control tries to decrease displacement to reduce torque demand. Can one control have priority over the other?

Ted

 

[HPost]

No Ted, they will not be in conflict. They do different things. The LS compensator sets the margin across the spool to which the pump will respond. The pressure cut off compensator sets the maximum outlet pressure. The torque controller takes oil after both of these and uses the pump pressure to move the swash angle.

The torque controller will back the pump off well before it reaches its corner power. If the pump is at low displacement and the pressure continues to rise, then the pressure cut off will limit the pressure.

If the controller and compensator settings are set too close together then the pump will be unstable, so the pressure cut off is usually set to the maximum working pressure of the pump. Well above the torque control setting.

This is the way that the Bosch Rexroth A10 pump works with the LA DS compensator.

Details of which can be found here...

https://www.boschrexroth.com/variou...ation=NET&/media/92703=&MEDIA_TYPE=Data Sheet

 

[Uncia_Uncia]

I started knowing lot of things from this discussion. I am still naive to the automobile industry,so please pardon me if I misunderstand anything.
Hydtools, I honestly didn't get you what you are trying to say in first two lines of answer in today's response. And as you mentioned, apart from cylinder,same PTO would be used for orbitrol steering pump from priority valve.
HPost, I really have to look into this Bosch Rexroth A10 pump and LS compensator and a torque limiter.