“Just put a seal kit in it” is one of the most common requests a hydraulic shop hears. Sometimes that really is the whole job. Often it is only the part of the job you can buy in a bag.
What a seal actually depends on
A rod seal wipes a polished chrome surface thousands of times a day. A piston seal holds pressure against a honed bore. Both depend completely on the metal they ride on. Put a new seal against a pitted rod and the pits cut it. Put a new piston seal in a scored barrel and oil slips past the scratches. The cylinder comes back leaking or drifting, often faster than before.
Symptoms that send a cylinder to the bench
- Oil leaking at the gland or rod
- A boom, bucket or mast that drifts down under load
- Weak or uneven stroke
- A rod that looks bent or has chipped chrome
- Scoring visible on an extended rod
- Worn pins, bushings or mounting eyes
- Dirty or dark oil in the circuit
Drift in particular can be the cylinder or a valve holding it. A cylinder bypass test should confirm which before anything is torn down.
What gets inspected, part by part
Rod and chrome
The rod is checked for straightness, scoring, pitting, corrosion and chrome that is peeling or worn through. Minor marks may polish out. Deep damage means a repair, re-chrome or new rod. A bent rod side-loads the gland and will destroy new seals quickly.
Barrel
The bore is checked for scoring, rust, glazing and wear that makes it oversize or out of round. Light marks can often be honed out, which also leaves the crosshatch new seals need. Deep scoring or dimensional wear needs machining or a new tube.
Gland and piston
These carry the rod and the piston in the bore. Worn bearing surfaces let things move sideways, which loads the seals unevenly. Seal grooves are checked for damage, and wear bands are replaced so metal doesn’t touch metal.
The oil and the machine
Scoring is often caused by contamination. If dirty oil put the marks there, a rebuilt cylinder on the same oil will get the same marks. The machine side counts too: overpressure, damaged mounts and misalignment can all kill a cylinder.
Choosing the right seals
Seals come in different materials and profiles for different pressures, temperatures and fluids. Matching a seal because it is the same size isn’t enough; it has to suit the cylinder’s working conditions.
Assembly and test
After the metal is right, the cylinder is assembled clean, lubricated and tested for leaks and full, smooth stroke before it goes back on the machine.
What this job teaches
The seal kit is the consumable. The rod, barrel, gland and piston are what decide whether it lasts. Inspecting and correcting those surfaces is the difference between a rebuild and a temporary fix.
Before you order a seal kit
Look at the extended rod in good light. Scoring, rust spots, or chipped chrome mean the cylinder needs more than seals. Tell your repair shop if the cylinder has been resealed before and how long it lasted.