If you were tuned in last month, you know about my lucky escape when a support tower snapped on the Blackcomb gondola. A great number of members sent their good wishes in response. So many in fact, it was not possible for me to reply to them all individually. And although it's largely inadequate, I'd like to acknowledge and thank those members here.
A few were quick to point out the failure wasn't anything to do with hydraulics. Actually it was. Not fluid power hydraulics. But the failure was hydraulic in nature.
The support pylon was a hollow cylinder or tube - designed to be hermetically sealed. But water leaked into the cylinder - and froze. The expansion in volume as the water froze created enough hydrostatic force to cause the tensile failure of a butt-weld joint - effectively jacking the top off the tower.
The fluid power hydraulics lesson to take away from this is very well summarized by one of our members who's used to working in freezing conditions:
"It seems that hydraulic principles and Mother Nature led to the failure of the tower. It definitely shows the raw power of a contained fluid under pressure in an enclosed cylinder and the effects of temperature on fluids.
We have seen a similar problem occur in our Snowcat fleet that happens when a tiller is brought in from the cold and removed. If the tiller is left in the warm shop and some hydraulic oil is not drained via the quick couplers, pressure rise caused by expansion of the warming fluid can ruin the motor seals, coupler seals and on a couple of occasions has split one of the motor cases."
Although not related to thermal expansion, another problem associated with contained fluid and which can arise in cold conditions is pressure intensification.
If the cap end of a double-acting hydraulic cylinder is pressurized and fluid cannot escape from the rod end - due to the oil being so cold it is below its pour point (or any other reason) this potentially dangerous phenomenon can result in catastrophic failure of the cylinder.
To demonstrate why pressure intensification can be so dangerous - and why we normally use meter-in flow control to control the extension speed of double-acting hydraulic cylinder, I've made a 6-minute video of a simulated model. It's explosive!
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