After approximately 16.000 landings the main landing gear of a Fokker 50 needs to be revised. This hollow plunger cylinder application is a solution to prolong the life span of its shock absorbers.
With each landing, the inner bore of the shock absorber wears out a little bit more. When the maximum is reached, the shock absorber becomes too weak and therefore unreliable. Its life span can be prolonged by inserting a metal tube, a sleeve.
To get the sleeve inside the shock absorber used to be a physical test for the service mechanics : The inner bore of the shock absorber measures 0,02 mm less than the sleeve. To expand the metal, the shock absorber was heated (150°C) first. Then, two men with heavy hammers slammed the sleeve in quickly and with brute force. This way, the sleeve could easily get stuck and it would then become totally useless.
Simply using one cylinder is not an option, since the stroke length is insufficient to insert the sleeve in one motion. Holmatro came up with a solution which does generate sufficient stroke length and pulling force. After heating the shock absorber, three coupled double acting hollow plunger cylinders pull the sleeve inside the
shock absorber in one controlled movement, by means of a tailor made iron bar.