Ask the Mechanic

Steve Thompson, Contributing Columnist

Removing a stump jumper (sometimes called a pan or round blade arm holder) from a rotary mower can be a very dangerous job. The round stump jumper (see photo accompanying this article) is on the machine to help protect the machine when encountering mounds of dirt, rocks, foreign objects or, yes, stumps. The stump jumper allows the swinging blades to pivot away from an object on impact, while the stump jumper skids over the obstruction.

When the stump jumper must be removed for gearbox repair, it can become a dangerous repair job. Sometimes the splined, tapered shaft that attaches the blade holder to the gearbox can easily be seized by rust because of its contact with moisture and debris.

The safest way to remove the stump jumper is to remove the nut and any washers under it. Next, take a pipe or thick bar and place it through the hole on the deck that is there to change blades. Pound on the arm part of the blade holder, and if you are lucky, it will drop off. However, most of the time, it's too tight. The next thing to try is turning the mower upside down and applying penetrating oil to the shaft, then letting it soak for several hours.

Here is where the safety issue begins. When the stuck blade holder assembly comes off -- whether it is from a puller, wedges driven under the blade arms, hitting the nut with a big hammer while the blade holder is under pressure, leaning up the mower and hitting it with a pipe through the blade removal opening or whatever it takes -- always leave the securing nut on the shaft and screwed down flush with the shaft end. That's because when it decides to come off, it can fly off, sound like a shotgun and hurt or even kill you. Again, always leave the nut attached to the output shaft of the gearbox when stump jumper assembly is exposed under pressure.

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-- Email Steve Thompson care of askthemechaniccolumn@gmail.com, and be sure to include your contact information and phone number.

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