This depiction summarizes a potentially catastrophic accident that occurs all too often when someone thinks he knows more about hydraulics than he actually does.

A Simple Procedure Gone Wrong

May 1, 2020
A student in a hydraulics lab was almost killed using a jerry-rigged cylinder rod extraction apparatus.

The lesson of the day was for students at an unnamed technical college to learn how to disassemble a hydraulic cylinder. This particular cylinder was of the round, welded design with the rod gland retained on the inside diameter of the cylinder tube.

The students apparently removed the gland retainer and initially attempted to remove the rod by laying the cylinder on the floor and attempting to slide the rod and gland out. They were unsuccessful because the gland appeared to be seized in place. It needed more force than they could muster to remove it.

With the instructor’s help, the students constructed a device they were certain would create sufficient force to extricate the stubborn rod gland. They started out with a large steel plate to which they welded an anchor. They secured the cap end of the cylinder to the anchor with a chain. The pulling-force would be generated by a hydraulic winch on the department’s tow truck. A student backed up the truck so its rear wheels rested on the plate. The idea was to have the tow truck provide the weight needed to hold the plate firmly against the cement floor while its winch would be used to extract the rod gland.

Aided by their instructor, the students attached the hook on the end of the winch’s cable to a chain, which was fastened to the cylinder’s rod clevis. The cylinder rod extraction apparatus was complete and the task of removing the rod was ready to begin.

When round-body, welded cylinders use a retaining ring to secure the rod gland, the gland might come out freely once the retaining ring has been removed. Then again, it might not.

Best-Laid Plans

One student sat in the driver’s seat while another operated the winch’s valves, located at the back of the tow truck. The other students and instructor watched intently as their experiment moved from planning to execution. One lucky student was assigned the task of operating the winch; another was given the task of sitting in the truck’s driver’s seat while keeping the brake pedal pressed.

The winch started to rotate slowly when the student activated the lever. The winch effortlessly lifted the cylinder into the vertical position, readying it for pulling action. The student in the driver's seat laid his right arm over the seat and twisted his body so he could observe through the rear window of the truck cab. The instructor gave a “thumbs-up”, so the valve operator activated the lever to set the winch in motion. The rear end of the tow-truck inched down, and the tires deflected as the winch torque increased to meet the resistance force offered by the stubborn rod gland.

The silence was broken by a thunderous explosion when the chain connecting the tow-hook to the rod clevis unexpectedly snapped. The action of the vehicle bouncing up, the tires springing back to normal, and the cable losing its tension caused the cable and hook assembly to recoil violently. The projectile unraveled from its sheave and took off in the direction of the vehicle’s rear window—through which the horrified student was peering.

The students watched helplessly as the projectile sailed through the vehicle’s rear window and out through the windshield. The student sitting behind the wheel froze as the steel projectile flew through the cab—missing his face by mere inches. Shards of broken glass became miniature projectiles popping as they struck every inch of the inside of the cab.

The broken hook assembly came to rest over the end of the tow-truck’s hood with the cable stretching from the winch through the vehicle’s cab and over the hood. The student in the driver’s seat—along with his classmates and the instructor—were fortunate to survive the incident without injury.

The lesson in “trial-and-error” was over. However, the lesson wasn’t a complete failure because the students learned how not to disassemble a hydraulic cylinder. Had the procedure been successful in freeing the gland, the students would have probably graduated from college having mistakenly learned that constructing a makeshift mechanism without knowing the strength of the relative materials is an acceptable way to remove a gland stuck in a cylinder. This is how unsafe maintenance practices perpetuate throughout industry and create accidents waiting to happen!

What Went Wrong?

The first attempt failed because rust had accumulated between the gland and cylinder wall, which caused the gland to seize. The most common—and certainly most dangerous—practice is to use compressed air to push the gland out of the cylinder tube. One of the worst, and most tragic incidents I investigated was related to compressed air.

Cylinder repair is best left to the experts. Handling, assembling, and disassembling cylinders (especially the round, welded type) require specialized fixtures and tools. Because none of these individuals were qualified to repair hydraulic equipment, they should have sent the cylinder to a certified hydraulic repair facility. The procedure that should’ve been followed is summarized in the accompanying box.

Fluid power safety doesn’t just happen—it has to be pursued.

Rory S. McLaren is the founder and director of the Fluid Power Safety Institute, West Valley City, Utah. For more information on fluid power safety, training, and related products, visit www.fluidpowersafety.com, or e-mail him at [email protected].

Caution: Rory McLaren and the Fluid Power Training Institute do everything possible to ensure that the information and drawings contained in these reports are accurate and that the suggested procedures are deemed safe and reliable. However, these are general recommendations only and might not be applicable to all situations. You must have your engineering and service departments read these recommendations and make the necessary changes for your specific conditions.

The Safe Way to Remove a Stubborn Gland

If you do not have the specialized training and the tools and fixtures needed to safely disassemble and assemble a hydraulic cylinder, send it to a reputable cylinder rebuild facility. However, there is a way to remove a stubborn gland safely.

1. Wear safety glasses.

2. Secure the cylinder on a suitable workbench. The cylinder must be slightly elevated at the cap end to purge the air when oil is poured into the open port.

3. Pull the cylinder rod until it is fully extended.

4. Install a plug into the cylinder’s rod-end port with at least the same pressure rating as the cylinder.

5. Place a receptacle at the rod end of the cylinder to catch the oil when the gland and rod become loose.

6. Make sure the rod is secured so it will not roll off the workbench once it comes out.

7. Remove the gland-retaining mechanism by following the respective cylinder’s service manual instructions.

8. Pour hydraulic fluid into the cap-end port of the cylinder until the oil is flush with the port.

9. Attach a hydraulic hand pump or portable power unit to the cylinder’s cap-end port. CAUTION: Never use a hydraulic pump or HPU without a pressure gauge.

10. Begin to pump oil (usually with a hand pump) into the cylinder while observing the pressure gauge.

11. It is safe to pump oil into the cylinder until the pressure reaches the maximum pressure rating of the cylinder or the system’s main pressure relief valve setting (the system from which the cylinder was removed).

If this process fails to push the rod and gland assembly out of the cylinder, do not make any further attempts to remove it. The cylinder must be sent to an authorized cylinder repair center for repair.

WARNING: Never attempt to remove the gland from a hydraulic cylinder with compressed air. Compressed air will turn the gland and rod assembly and or the cylinder tube into a steel “missile”. Workers have been severely injured and killed while attempting to disassemble hydraulic cylinder with compressed air.

Continue Reading

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