Cylinder Velocity

[SpenceQ]

I'm designing a backhoe for an IH farm tractor with Aux Hydraulics, I've come down to a calculation for cylinder velocity of 2.8 seconds which I find much too quick. The shock on the cylinder would be excessive, let alone on the system itself. Can anyone see where my calculation could have gone wrong here:

Cycle times for cylinder strokes on unshafted side:

(cylinder 14inches long and 3inches wide)

3.1415 * (1.5*1.5) * 14

98.96 cu. inches

(volume in gals for cylinder.)

constant: 231 cu. in/gal

98.96 / 231 = .428 gals

(Assuming 9 GPM/2100 psi standard tractor pump)

.428/9 = .047 minutes

60 sec. per min. * .047 = 2.8 seconds

I could put a restricter in the line, but that would heat the line. I have never seen such a pump in top condition move a 3in cylinder that fast, and the IH tractor 3PH system in the subject tractor has a standard 3in cylinder. The 3PH linkage is quick, but not instantaneous. I will flow meter the system, but I know it's in good condition. All in all, the calculation seems OK, and 9 GPM for agricultural pumps is standard.

Any help would be appreciated.

Spence

 

[MikeHalloran]

Your math looks okay to me.

That's the speed the cylinder could achieve if there's only one cylinder being fed at the time, and the control valve were instantly moved to full open and the resulting load/inertia induced pressure was not sufficient to open a relief valve.

Uh, so what?

You wouldn't normally do that ... okay, I've seen it done, e.g. for hammering a bucket into hard ground, etc.

If you're concerned about the shock load on structure and seals ... well, even if you're not, you should include cross- port and similar relief valves everywhere anyway.



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi spenceQ

Shouldn't the area of the cylinder be divided by 4 in the first line before multiplying by length to find volume.

regards desertfox

 

[desertfox]

Hi

sorry you have used radius thought it was dia.


regards desertfox

 

[SpenceQ]

Thanks all for the help.

Spence

 

[rb1957]

i think mike had a good comment.

the math assumes everything is constant and going full bore, sure a 9gpm pump will empty a 1/2 gallon cyclinder in 1/18 of a minute, about 3 sec.

but reality sets in ...
once the cyclinder is full, and the pump running, what size opening is in the cyclinder (maybe the pump is being "throttled" by the plumbing ? this'll tell you something of the flow rates ... this isn't going to be a step change (ie it'll take a bit of time to adopt steady state conditions). is the cyclinder feed assisted by gravity ? or is the pump trying the suck the contents out of the cyclinder ? (this'll be a much slower process).

just because you have a 9gpm pump doesn't mean it's pumping 9gpm all the time.

does that help any ?

 

[satchmo]

What about regen?