Hydrostatic Transmission by Mike Tilby

[Ronald WagnerParticipant @ronaldwagner57800]

I was very interested in the recent article by Mike Tilby regarding hydrostatic transmission, in large part because hydraulics have always been a bit of a mystery to me.

I was trying to be sure I understood the fundamentals and was being caught up short by the diagram in “Box 1”.  I finally discovered why things were not adding up correctly.  I really would have liked that diagram to include the fundamental equations, but I suppose maybe those aren’t necessary for the learned readers.

What I concluded was that the text included in the box was correct, but the diagram’s final output value was not.  Assuming the pressure in the large cylinder is 255 psi and the diameter of the cylinder was 5in, then the output force should be:

F = p * A   and   A = pi * ( 5 / 2 )^2 or 19.63 sq in

F = 255 * 19.6

F = ~5006 lbs and not the 1265 shown in the diagram.

This then agrees with the text’s statement of 100x input force of 50lbs

Ron

[Ronald WagnerParticipant @ronaldwagner57800]

I perhaps should have made clear that I did appreciate the article – I’m just a little slow on my math.

And if it saves anyone some typing, there are the youtube links mentioned in the references:

 

[noel shelleyParticipant @noelshelley55608]

Don’t recall the article BUT your 5006 pounds force is correct for a pressure in the cylinder of 255psi. But where did the 255 come from ? A simple lever and small cylinder ? Small cylinders and lever type pumps will raise 10,000PSI with fair ease, so your 5″ cylinder will produce 87.5 tons – as just another example. I often use a 3″ piston jack that is rated at 25 tonnes.  Noel.

May be I should point out that whilst the piston may produce a force of 87.5 tons the piston rod may well not be able to take this load. Ordinary systems often work at 2500psi for power operation, eg tractors etc. From memory, flow in gallons/min  X  pressure in PSI divided by 1428 gives the HP required.

Flow is the speed at which the system will operate.

[Ronald WagnerParticipant @ronaldwagner57800]

Yes, in the article, the 255psi was generated with a much smaller, lever actuated cylinder.

I’m away from the article right now, but the diagram was something like a 1:5 lever acting on a cylinder.  A 10# force on the lever results in 50# on the cylinder.

The cylinder had a 0.5″ diameter, for an area of .196 sq in.  50/.196 = ~255psi

 

[noel shelleyParticipant @noelshelley55608]

You seem to have got it ! The numbers add up and would work for a small hand pump on a model but for a small hydraulic jack a mechanical advantage of 15 would be more likely and on a bigger one 25. When one then adds the hydraulic advantage one soon starts to measure in tons. Noel.

[duncan webster 1Participant @duncanwebster1]

I once tried to get a loctited wheel off its axle using a hydraulic jack. It wouldn’t budge so I put a length of scaffold pole over the handle and leaned on it. The cylinder burst, depositing me on the floor

[duncan webster 1]

On duncan webster 1 Said:

I once tried to get a loctited wheel off its axle using a hydraulic jack. It wouldn’t budge so I put a length of scaffold pole over the handle and leaned on it. The cylinder burst, depositing me on the floor

the relief valve should have stopped that happening

[noel shelleyParticipant @noelshelley55608]

Small vertical hydraulic jacks do not tend to have a relief valve – relying on the supplied handle as being able to keep the device within it’s design limits. When you reach for the extension tube/bar then all bets are off . The risk of a simple overload valve letting go with the load on and someone under the load is a bigger risk. Even the cheap trolley/floor jacks have a relief valve. BUT we all know that such a device can easily be wound in. History is full of tales of disaster following the unauthorised meddling with valves !

Steam engineers talk in tens or hundreds of PSI, Hydraulic engineers talk in thousands of PSI. If Duncan would care to give figures for the pump piston diameter and the mechanical advantage, plus a clue as to the force he was applying, it might frighten him and be interesting for us ?  Noel.

PS the process of hydroforming is interesting.

[duncan webster 1Participant @duncanwebster1]

I’ve long since dumped the split cylinder and have no idea of the mechanical advantage. The only name on the outer case is Sprint 2, and the ram is 24mm diameter. If it were a 1te capacity this would equate to 22 MPa, about 3300 psi, pro rata for higher capacity,  I probably doubled the length of the lever, and had my full (not inconsiderable) weight on it

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