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Mechanics Corner: Always keep power take-off (PTO) safety in mind

Mechanics Corner Published on 19 July 2012

The power take-off (PTO) shaft is an efficient means of transferring mechanical power between farm tractors and implements.

This power transfer system helped to revolutionize North American agriculture during the 1930s. It is also one of the oldest and most persistent hazards associated with farm machinery. This article discusses several aspects of PTO safety.

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PTO hazards
Power take-off (PTO) stub: The tractor’s stub shaft, often called the PTO, transfers power from the tractor to the PTO-driven machine or implement. Power transfer is accomplished by connecting a drive shaft from the machinery to the tractor’s PTO stub shaft.

The PTO and drive shaft rotate at 540 rpm (nine times per second) or 1,000 rpm (16.6 times per second) when operating at full recommended speed. At all speeds, they rotate in proportion to the speed of the tractor engine.

Most accidents involving PTO stubs stem from clothing caught by an engaged but unguarded PTO stub.

The reasons a PTO stub may be left engaged include the operator forgetting or otherwise not being aware the PTO clutch is engaged, seeing the PTO stub spinning but not considering it dangerous enough to disengage or the operator is involved in a work activity requiring PTO operation.

Boot laces, pant legs, overalls and coveralls, sweat shirts and windbreakers are clothing items that can become caught and wrapped around a spinning PTO stub shaft.

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Power take-off (PTO) drivelines: The PTO driveline hazard (sometimes identified as a machinery “wrapping point” hazard) is one of the oldest and most common farm machinery hazards and refers specifically to the part of the implement (machine) drive shaft that connects to the tractor. This drive shaft is known as the implement input driveline (IID).

The entire IID shaft is a wrapping point hazard if the IID is completely unshielded. If the IID shaft is partly guarded, the shielding is usually over the straight part of the shaft, leaving the universal joints, the PTO connection (the front connector) and the implement input connection (IIC, the rear connector) as the wrapping point hazards.

Protruding pins and bolts used as connection locking devices are particularly adept at snagging clothing. If clothing doesn’t tear or rip away, as it sometimes does for the fortunate, a person’s limb or body may begin to wrap with the clothing.

Even when wrapping doesn’t occur, the affected part may become compressed so tightly by the clothing and shaft that the person is trapped against the shaft.

The machine’s IID shaft is coupled to the tractor’s PTO stub. Therefore, it too rotates at either 540 rpm (nine times per second) or 1,000 rpm (16.6 times per second) when at full recommended speed. At these speeds, clothing is pulled around the IID shaft much quicker than a person can pull back or take evasive action.

Many IID shaft entanglements happen while the shaft is turning at one-half or one-quarter of recommended operating speed.

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This may be the situation on occasions when the tractor has been stopped but not turned off and the PTO is left engaged. Even at slower speeds, once caught by a IID shaft, a person may not have time for evasive action.

A 540 rpm shaft makes over two complete revolutions per second when operating at one-quarter speed. Even with a relatively quick reaction time of five-tenths of a second, the wrapping action has begun. Once wrapping begins, the person instinctively tries to pull away.

This action simply results in a tighter, more binding wrap. The 1,000 rpm shaft roughly cuts in half the opportunity for evasive action.

PTO safety practices
Though not always easy or convenient, there are several ways to reduce the risk of PTO injury incidents. These safety practices offer protection from the most common types of PTO entanglements.

• Make it a specific point to keep all components of PTO systems shielded and guarded.

• Regularly test driveline guards by spinning or rotating them to ensure they have not become stuck to the shaft.

• Disengage the PTO and shut off the tractor before dismounting to clean, repair, service or adjust machinery.

• Walk around tractors and machinery rather than stepping over a rotating shaft.

• Keep universal joints in phase. (Check the operator’s manual or with a farm implement dealer if you do not understand what this means).

• Always use the driveline recommended for your machine. Never switch drivelines among different machines.

• Position the tractor’s drawbar properly for each machine used. This will help prevent driveline stress and separation on uneven terrain and in tight turns.

• Reduce PTO shaft abuse by observing the following: Avoid tight turns that pinch rotating shafts between the tractor and machine, keep excessive telescoping to a minimum, engage power to the shaft gradually and avoid overtightening of slip clutches on PTO-driven machines. PD

—Excerpts from Penn State Agricultural Safety and Health Program Fact Sheet

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