This is part 2 in a series on the importance of following good safety protocol in fluid power system maintenance and design. It highlights real-life examples of the dangers and injuries that can occur and provides advice on preventing them. Find part 1 here; part 3 here; part 4 here; part 5 here; and part 6 here.
Scenario: Ron, a maintenance worker, was trying to break a conveyor free after it unexpectedly jammed during production. To do this, he made a makeshift lever mechanism that consisted of a pipe wrench with a long handle, and his own body weight. He clamped the jaws of the pipe wrench around the exposed conveyor drive shaft, and straddled it with his body.
Ron then proceeded to bounce on the end of the long pipe wrench handle, in an attempt to free the conveyor. As he did, the conveyor unexpectedly broke loose. The handle beneath him whipped him high into the air, Figure 1. Ron struck the ground several feet away, and sustained fatal neck and head injuries.
Events that led to the accident
Ron reportedly received a call from the machine operator that the loaded conveyor had unexpectedly stopped during production. During his initial investigation, he found no obvious signs of physical damage. However, there were areas along the conveyor where visibility was extremely poor. Thus, he could not determine if the problem was mechanical or hydraulic.
He checked the pressure gauge on the power unit, and saw that it recorded full system pressure. Through a series of checks, he concluded that the conveyor was jammed. He decided to see if he could force it to rotate with the objective of breaking it loose. The only device that he felt could accomplish this was a makeshift leverage system comprised of a pipe wrench with a long shaft — and his body weight.
During his initial attempt to free the conveyor, the power unit was switched off and locked out. He fastened the jaws of the pipe wrench firmly around the exposed conveyor drive shaft. With his full body weight, he then bounced on the handle, but the conveyor showed no signs of moving.
Ron then advanced his trial-and-error troubleshooting technique to the next level. He decided to let the hydraulic motor torque to assist him, while he continued to apply torque using his makeshift lever mechanism. He started the power unit and placed the motor rotation selector in manual override position. Next, he asked a colleague to assist him by pushing the manual override pushpin in on the solenoid-operated directional control valve, thereby applying maximum system pressure against the motor port.
Once again, he fastened the pipe wrench jaws around the conveyor drive shaft, and began his ritual of straddling the handle with his body and bouncing on it with his full body weight. After a few attempts, the conveyor unexpectedly broke free. With his body straddled across the handle — and the sharp teeth of the pipe wrench jaws firmly clenching the steel conveyor drive shaft — the pipe wrench handle whipped around, catapulting him high into the air, as if he were a rag doll. The impact of hitting the ground caused Ron to suffer fatal injuries.
Contributing factors to the accident
Improper procedure — the victim was using nothing more than trial-and-error methods to find the problem with the hydraulic motor/conveyor system.
Inexperience — although Ron had worked more than 20 years as a maintenance technician, he had no formal hydraulics training and had never received training on safely testing hydraulic motors. Additionally, training for his supervisors did not include key safety issues vital to this procedure.
Unreported incidents — a similar incident had occurred while attempting to test a hydraulic motor/conveyor. However, because it resulted in a non-injury accident, it had not been reported to or investigated by management.
Steps to prevent a similar accident
Training — ensure that every person who works on and around hydraulic equipment receives technical training — do not take on-the-job experience into consideration! If you have never received formal hydraulic training, make an application to do so immediately. If you are asked to perform a job that you are unfamiliar with, tell your supervisor.
Procedure — Never attempt to stall a hydraulic motor or force a hydraulic motor to rotate while it is under pressure. Motor torque is a function of pressure and displacement, and can be grossly underestimated. Do not use cables, chains, or other pry-bars to stall a hydraulic motor. High motor torque could cause a stretched cable or chain to snap and recoil. A foreign object could get bound up in a rotating mechanism, and may fly out unexpectedly if the mechanism is reversed.
Job Safety Breakdowns — have your safety department compose a Job Safety Breakdown for all unconventional tasks. These should be made readily available to all personnel.
Diagnostic instrumentation — management should always provide employees with the proper tools for the job. Flow meters and pressure gages should be made available for testing hydraulic motors.
Safety — discuss hydraulic safety in your regular safety meetings. Safety professionals should undergo hydraulic safety training so they can recognize potential hazards and support maintenance personnel and operators with hydraulic safety issues.
During an investigation into the cause of this accident, it was determined that the conveyor jammed because a piece of the material it was transporting became wedged between the conveyor and the conveyor frame assembly.
When a hydraulic motor unexpectedly stops, here is a brief outline of what you can do to determine if the problem is hydraulic or mechanical (follow your company’s lockout protocol when installing or removing diagnostic instrumentation):
1. Check the oil level.
2. Check pump coupling for damage.
3. Install a pressure gauge at the pump outlet port — if full pressure, proceed.
4. Install a pressure gauge at the directional control valve port — if full pressure, proceed.
5. Install a pressure gauge at the directional control valve outlet port — if full pressure, proceed.
6. Install a pressure gauge at the motor inlet port — if no pressure, back up to obstruction; if full pressure, proceed.
7. Install a pressure gauge at the motor outlet port. If no pressure, look for mechanical obstruction or motor damage. If full pressure, there is an obstruction down line.
Lastly, note that if a pressure compensated pump is being used, start troubleshooting at the outlet port of the directional control valve.
If all else fails, ask for help. It is always wise to err on the side of safety!
Figure 1. Untrained maintenance worker begins his ill-conceived testing procedure.
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 Fluid Power Training Institute is not responsible for actions taken by untrained or unauthorized persons. All hydraulic system service, repair, and troubleshooting should be conducted only by trained, authorized personnel.