The effect of temperature on static hydraulic pressure

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In Insider Secrets to Hydraulics I discuss what it takes to become proficient at hydraulic troubleshooting. But there's another pre-requisite you won't find in the job description: the ability to educate and convince the most ardent sceptics - usually engineers with limited hydraulics knowledge. This story, sent to me by Joachim Renner, one of our members from Germany, is a great illustration:

Our company supplied eight double-acting hydraulic cylinders for an auxiliary function on a tunnelling machine. The cylinders had the following dimensions:

Piston diameter 140 mm
Rod diameter 90 mm
Stroke 600 mm

During machine commissioning we got a complaint that none of the hydraulic cylinders were holding pressure. All cylinders had been tested prior to delivery. Despite this, it was plausible one cylinder had failed - but not all eight!

All the hydraulic cylinders were removed and returned to us for debugging. Each was pressure tested first. And no defect was detected in any cylinder. So we sent them back to our client - with test reports.

After a couple of days I got a call from a very angry customer. All of the hydraulic cylinders were still not tight!

What could I do?

I arranged to meet the client's technical people, on-site the following day.

During static testing for final acceptance, the hydraulic cylinders kept failing. To demonstrate the problem, the cylinders were extended to the internal stop and pressurized to 250 bar. A decrease in static pressure could be observed on the cylinder circuit's pressure gauge. Eventually the pressure dropped to zero. All cylinders were fitted with pilot operated check valves at the cap-end port.

I asked for some technical background on the machine and was informed the designer had avoided using a separate pump circuit for this function by tapping the main circuit (350 bar operating pressure) with a 2 millimeter orifice and pressure reducing valve (250 bar). Hand-operated directional control valves were used for cylinder control.

The pressure required for unloaded cylinder extension was approximately 10 bar. I realized the pressure drop from 350 bar to approximately 20 bar was heating the hydraulic oil being supplied to the cylinders. And this was the root of the problem.

To demonstrate the faultless function of the hydraulic cylinders I installed a ball-valve and pressure gauge at the cap-end port of the cylinder - to eliminate all other control elements.

The cylinders were extended to the internal stop and pressurized to 250 bar. The ball valve was closed and the rod-end port connection was removed. As the hydraulic oil and cylinder body started cooling down, pressure decreased slowly to zero, although no leaking could be detected at the rod-end port.

I explained to the engineers on site that the cause of their 'apparent' problem was the heating and subsequent cooling of the oil being supplied to the hydraulic cylinders during static testing.

But they still didn't believe me and remained of the view that the problem was a result of leaking seals.

So to prove the point, I got a hot-air gun - normally used for packaging wrap and started to gently heat the hydraulic cylinder. As I expected, cylinder pressure started to increase.

Seeing is believing ... and at last the blind could see.

Related articles:

How to test hydraulic cylinders
How to solve hydraulic cylinder squeal
Would you pass this simple hydraulics test?

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