Slowing down a hydraulic ram 1

[Rick A]

Hi all,
New guy seeking advice. I have a small Gresen pump out of a bearing press running a 3" x 12" ram and I understood I could slow the ram speed by plumbing in a valve after the pump to modulate oil flow. It doesn't change the speed until just before I close it and then it stops the pump motor. There's no relief valve on the pump that I can see.
Any suggestions? Thank you.

Rick A

 

[BrianPetersen]

Odds are, it's a fixed-displacement pump, which means it's going to try to pump that fixed-displacement (which translates into fixed cylinder speed) no matter what, up until the pressure across the pump becomes excessive and either (1) stalls what's driving the pump or (2) breaks something.

So, to slow down the cylinder motion, you either slow down the pump (e.g. by driving the motor at a reduced frequency and thus speed through a VFD - the complicated and accurate but efficient way), or give part of the fluid someplace else to go (e.g. by installing a relief valve before your adjustable flow-control restriction - the cheap and easy to adjust but inefficient and fluid-heating way), or use a different pump of a variable-displacement type.

Which way makes sense, depends on your application.

 

[TugboatEng]

This is a funny case, I think you'll find all options cost about the same. You can use a VFD to control the speed of the pump. You can run the pump against a relief valve and portion off some flow to your ram. You can replace with a variable displacement pump. You must compare total cost of every option.

 

[Rick A]

Thank you both. My first thought was to slow the pump, which I did via a double reduction of belts and pulleys, but it then didn't pump at all. Now I know that pumps have a minimum speed! Relief valve sounds like the way to go. It's for intermittent use so efficiency and heating aren't priorities.

Rick A

 

[LittleInch]

You were lucky.

Normally it is vital for a fixed displacement pump to have a relief valve. The pressure you can get before the pump stalls can be huge and you can easily burst pipes and break equipment.

Make sure any PD type pump has a relief valve upstream off any valve, isolation or control.

If you don't you could easily have injured yourself or others.

Learn from you experience or you won't live to make any more.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Rick A]

In digging deeper I found a diagram of the control valve unit (it's 60 years old) and learned that it incorporates a relief valve, so I suspect it opens at a higher pressure than the motor (not original) can produce. Would an alternative be to run a line from the control valve outlet to the return to the tank with a valve in it to bleed off oil and slow the ram? Thank you.

Rick A

 

[LittleInch]

Yes but you seem to be going off at random.

Get all the information about the pump and controller first.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

Install a priority flow control. It bypasses excess flow at operating pressure, not the higher relief valve pressure.

Ted

 

[Rick A]

I put a tee in the line to the ram and connected one leg of it to the return through a needle valve. When I crack the valve it bleeds off some of the fluid to the ram and slows it down. Simple and done with what I had on hand.

Thank you BrianPeterson, TugboatEng, and hydtools for your inputs.

Rick A