(Ref. No: MSB-002)

What's this Maintenance Safety Bulletin about? A number of people have either been mutilated, or lost their lives because they did not realize that dual hydraulic cylinders used for lifting, are generally connected in parallel.
What is "parallel?" The easiest way to determine when hydraulic components are connected in parallel (versus series), is to determine if, as you follow the oil transmission lines, you can use the word "OR."
For example, if you follow this single transmission line, it splits into two. Thus the oil can flow to the left "OR" to the right.
If two cylinders are connected in parallel, and one directional control valve is used to operate both cylinders, then somewhere along the line there has to be a split that directs the oil to both cylinders in parallel.
How does the fact that the cylinders are connected in parallel cause a problem? Here is an example: A motor mechanic with no training in hydraulics was assigned the task to remove one of the two lift cylinders on a front-end loader. He raised the bucket several feet in the air and walked in underneath the unsupported bucket to remove the defective cylinder. He thought that the adjacent cylinder would continue to hold the bucket securely in the raised position, while the cylinder was being serviced. When he disconnected the hose from the cylinder, the entire assembly unexpectedly collapsed crushing him. As a result of the accident he lost his right arm and shoulder. Here is another example: (see original source of accident report) A farmer's corn planter had a flat tire. To remove the wheel, he placed a jack under the axle and attempted to lift it off the ground. However, the machine was equipped was a hydraulic height control system that would not allow the wheel to lift off the ground without lifting the entire machine - the jack was not capable of this! The farmer apparently figured out that the only way he could lift the wheel off the ground without lifting the entire machine, was to disconnect the hose from the height control cylinder on that wheel. He apparently thought that the cylinder on the wheel adjacent to the one he was working on would hold the machine up when he disconnected the hose. However, as with the previous example, the cylinders were connected in parallel. When he disconnected the hose from the cylinder, the machine unexpectedly collapsed on him, killing him instantly. The farmer evidently had no formal training in hydraulics. What immediate action must I take? 1. Never work underneath, or on top of, unsupported loads. Even if, in both these cases, the cylinders were connected individually, the conditions would still have been extremely unsafe. 2. Bear in mind that neither of these people had prior training in hydraulics. Training is the first step in preventing accidents. It empowers people to make safety-based decisions. If you are not formally trained in hydraulics it's time to sit down with your supervisor and/or safety manager and ask for training - get a commitment and make sure they, and you, stick to it! 3. Talk about this type of problem in your safety meetings, and also with your colleagues. If you see or hear of a colleague that might be doing something similar to what these unfortunate folks were doing, help them.

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