How Do You Calculate Hydraulic Cylinder Speed and Force with Dual Cylinders?

[grandnat_6]

Hi,

I've been reading up on hydraulic fluid power design to design a lifting system for a loader bucket I'm building for my 20Hp tractor. I know how to calculate the force/ speed of one cylinder extending; but I am a little lost as far as using two cylinders. If I have a total weight of 2000 lbs to be lifted and have a 2" cylinder with an extension of 20" on one arm and another 2" cylinder with an extension of 20" on the other arm, both cylinders are actuated by a control valve with both cylinders sharing the same hose with a "Tee" to direct fluid to both cylinders. I know the one with the least resistance will extend first.

When calculating; do I use the area of one of the 2" cylinders and an extension of 40"? Or do I use the total area of both cylinders and the extension of 40"? I'm a little confused because I've seen sales brochures with stated PSI, cylinder size, and extend times and I can not get them to match my calculations.

Thanks,

Grandnat_6

 

[CWatters]

In the set up you describe the cylinders are effectively in "series". That set up would be used when you want to increase the extension compared to using one cylinder. It doesn't increase the load capacity/force. Use the area of one cylinder. The total speed should be the same as one cylinder (if limited by the flow rate of the pump).

If the cylinders were in parallel (two cylinders moving the same arm) that would increase load capacity. Use twice the area to calculate the available force. The speed would be half (if limited by the flow rate from the pump).

 

[JBA]

Assuming a positive displacement hydraulic pump then the lightest loaded cylinder will extend at the pump volume delivery volume rate until its loading is equivalent to that of the other cylinder; which, at that point, both cylinders will extend at the same rate until the first cylinder hits it full extension, after which the second cylinder will continue to extend at the pump delivery rate until it hits the end of its stroke (and the pressure bypass valve opens) or you close your control valve at some point along the way.

The biggest risk in hooking up the cylinders as you are proposing is that a large eccentric loading can twist your bucket assembly until the arm bearings begin to bind and possibly become damaged; or, the twisting stress on the assembly connections become high enough to cause a fracture failure.

 

[grandnat_6]

Saying parallel would have been a better choice of words for my writing. It would use a positive displacement pump. I did think of the situation JBA described. Would a pressure compensating proportioning flow divider to solve this issue?

Thanks again, you both are very helpful.

 

[JBA]

You should talk to a hydraulic equipment supplier and/or servicing technician regarding the best option(s) for dealing with the eccentric load problem.

 

[grandnat_6]

When I get down to purchasing I will ask them about eccentric loads at that time. Thank you.