Posted by charles todd on February 05, 2009 at 18:43:14 from (205.242.95.141):
In Reply to: PTO slip clutch posted by Gary Hutchinson on February 05, 2009 at 08:14:23:
In the deep south we have a few inches of snow about every 10-15 years... :( But we have lots of grasses and vegitation, even some in the winter. On brush cutters a slip clutch wins my vote over a shear. The only drawback is proper use and PM. I say this because surface rust will stick them. My father trashed a gear box on a 10' two blade and a side box on a 15' bat-wing due to improper PM.
On a slip clutch at the first of the season it is wise to loosen the bolts to release the tension on the springs. Then engage the PTO and let it slip a bit. Then follow the OEM specs on how much to tension the springs. With 6 to 8 springs, a 1/8" can mean a lot. Follow this each season and a slip clutch is a wonderful thing.
Now for shear pins. If the engineer that designed the machine did their homework they calculated the shear point close to the max torque rating of the reducer. It may be a pin with a reduced center or simply a GR 2 or GR 5 bolt.
At the manufacturing facility I am employeed at has overhead monorails. One has over 5200' of chain with six VFD drives daisy-chained in a master/slave arrangement. The VFD's manage the torque rise through communication with each other to provide "droop" to the peaking drive. Sometimes we shear a pin. The drives go through 800:1 reducers and I calculated the total torque once and it was about 64,000 ft/lbs for all six drives pulling.
Our shear pins are 1" stock with the center turned down to a certain diameter. It is known that by how much is removed, vs material used, vs mechanics of the system when it will fail. For instance the shear pins on the two drives through the powder coat oven are thicker that the ones idling through the plant. The heat and multiple turns provide more resistance.
I know I got off topic there. Basically what I am saying is, a properly designed system with a shear pin should shear at a repeatable torque load. A slip clutch is just that, it slips under excess load then resumes when the load clears or keeps slipping until the load is removed. Personally I prefer a slip clutch on anything. Be it a mower, baler, mixer-mill, ect. I do not like to have to carry a handfull of shear pins if I think I may be working the equipment in heavy loads. Let it slip, back off, let it clear, and resume.
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