Plunger from hydraulic cylinder becomes an 18kg projectile

Mines safety alert no. 63 | 25 October 2001 | Version 1

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A recently serviced 100 ton double acting hydraulic cylinder and its associated pump were being tested on the floor of the workshop. The advance and retract hoses were connected and hydraulic pressure of 6000psi was applied to the advance port. The plunger extended only a few millimetres and then shot out of the cylinder and up through the workshop roof before landing outside. This projectile weighed 18kg.


100 ton double acting hydraulic cylinder.


Hydraulic pressure.


The plunger and piston were originally made from a single piece of material. The service provider decided on his own initiative to replace the plunger because it was scored. He did so by machining the plunger free from the piston and welding the new plunger to the piston as the photograph of the damaged parts shows.

When the unit was tested in the workshop the retracting hose was not fully tightened so that the check valve in one or both halves of the fitting were still closed. Also a safety valve should have been fitted to the retract chamber but this had on some previous occasion been removed and its port blanked off.

Consequently when the hydraulic pump pressure was applied to the advance port an overpressure situation occurred in the top retract chamber of the cylinder. This resulted in shearing off the piston from the plunger which was then ejected from the cylinder with tremendous force.


The pressure applied to the advance port was well within the design pressure of 10000psi. The service provider was not authorised or trained by the manufacturer to carry out this work.

Hydraulic pressure

Owners of equipment should ensure that contractors servicing the equipment are competent and have the necessary resources whether they are on or off site.

Whenever modifications are made to plant it must remain fit for the intended purpose. Modifications to this type of equipment should not be carried out without approval of the manufacturer.

Authorised by Frank Jones - Regional Inspector of Mines (Northern)

Issued by the Queensland Department of Natural Resources and Mines

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