hydraulic motor feasibility 2

[engrnick]

I have a question for a design concept for a project i'm working on. I'm not a hydraulics guy, but if this works, we'll have to pull one on. I just want to make sure this design concept makes sense.

We have a platform providing 3HP, 3000psi hydraulic. we are designing a concept that uses 5 hydraulic motors instead of electric to drive an internal system.

Questions:
1) Can a hydraulic motor be cycled on and off quickly without damage to the motor?
2) Our current concept uses stepper motors to accurately control speeds and position - how can we do this with hydraulic motors?
3) We need the motors to spin at exactly the same rates, around 150 RPM, around 40inlbs. Is this possible?
4) How heavy do you estimate the motors will be, 100lbs or greater?
5) is there a way to prevent drop in speed if 3 motors are running and then turning on one more motor?

Thanks, I'm sorry for the questions, but I need to determine if my project needs budget for a hydraulic guy, or if we keep down the path using electric motors. Using hydraulic would simplify the feasibility of the project a bit more.
Nick

 

[MikeHalloran]

WRT (3), they'll turn at the exact same rate only if geared to each other.

Mike Halloran
Pembroke Pines, FL, USA

 

[engrnick]

That's what I was wondering - is there a method to control them so they spin the same rate without gearing them to each other? I need them to spin independently direction wise, and then sync together occasionally. This is why I'm leaning more towards electric motors.

 

[MikeHalloran]

The second generation of CNC equipment used stepping motors plus hydraulic motors as amplifiers. They still lost a step once in a while, so they lost the market to servomotors. I don't know if you can still get the hardware to do it that way.

You could put encoders on the hydraulic motors, control them with servovalves (or maybe proportional valves), close the loop, and do what you want, but you're looking at a pretty complex system.

Lots of traditionally hydraulic equipment like tube benders is transitioning to all-electric as high power servomotors and their controls become more competitively priced. ... and environmental and safety regulations make hydraulics, and leaks, more of an issue.



Mike Halloran
Pembroke Pines, FL, USA

 

[hydromech]

1) Can a hydraulic motor be cycled on and off quickly without damage to the motor?

There will be an inherent latency within the system and you may find that your motor speed averages out...kinda like PWM.

2) Our current concept uses stepper motors to accurately control speeds and position - how can we do this with hydraulic motors?

You'll get a smaller motor package for the same power output, but getting the same level of control will require a control algorithm.

3) We need the motors to spin at exactly the same rates, around 150 RPM, around 40inlbs. Is this possible?

The manufacturing tolerances with the different motors means they will have a slightly different RPM. As suggested, this can be overcome by setting up closed loop control to synchronise the motor speed.

4) How heavy do you estimate the motors will be, 100lbs or greater?

You only have 3HP at 3000 PSI. You can only use small motors...using 5 together means they will be very small. Your flow rate can only be approx 0.5 GPM, only 0.3 GPM if you consider system efficiencies.

5) is there a way to prevent drop in speed if 3 motors are running and then turning on one more motor?

Yes, you can use an accumulator to supplement the supply for short periods. However, depending on what you are trying to achieve, the accumulator may be quite big.

Adrian

 

[MikeHalloran]

WRT (5), if your supply is limiting your flow through 3 motors, then adding a fourth motor will necessarily cause the other three to slow down.
Conversely, if the supply is more than big enough to supply all the motors at the flow rate you need, then you will be dumping high pressure oil through a relief valve, which is noisy and inefficient.
On the third hand, a variable displacement pump in the supply can be controlled to supply only as much oil as needed, saving considerable energy.

WRT (1), if you cycle a hydraulic motor on/off at stepping motor pulse rates, the system will develop acoustic noise, and the fatigue life of the tubing and whatever you are driving becomes an issue for consideration. It doesn't work quite as nicely as PWM of electric motors; when you stop the flow, the motor stops instantly or nearly so.
You can cycle a hydraulic motor on/off at rates that would overheat an electric motor, e.g. hundreds of times an hour, with no damage to the motor or a properly designed system.


If you can't reveal more detail about what you're trying to do, e.g. for commercial reasons, then you definitely need a hydraulics person on premises.



Mike Halloran
Pembroke Pines, FL, USA

 

[engrnick]

Thanks guys, you're a big help.
It looks like it's not feasible for our application. These are all considerations that will cause more headaches than it's worth - we need it to be relatively lightweight, an accumulator will add weight. Also it will be stepping frequently, forward and reversew quickly, so an electric looks like the better choice here.

I can't share more about it due to commercial reasons. I was just trying to figure out if we should bring someone on board.
This helped a lot.
Nick