Help on a pulling sled.

dmiller

Member
Our FFA chapter has hosted a lawn mower tractor pull the past few years. The sled we made last year worked fine but when the weight box got up about 70% of the ramp the right side of the winch axle broke (broke the weld where we had shortened the axle to get the sled to be narrower to fit the small lawn tractors). We fixed it up, though it was very strong and this year it broke again.
Any suggestions on how to keep this from happening again?
a190087.jpg
 
Just some random thoughts from an old retired sled operator. Does the box run on rollers or does it just slide on the rails? Rollers move easier than slides. The cable pull looks as thought it wraps around the hubs, the further you pull the larger diameter the pulley becomes. That makes the box travel faster and puts more torque on your drive train. You might try to load the pan more, add some counter weight on the back and lighten the box. Sometimes a correctly balanced sled that is heavier will start and pull easier. The front pulleys need to be raised up where the cable runs in the same plane as the rails. If the cable is pulling down the higher up the rail the box goes the more angle is put on the cable. Is the differential on the front welded? Need to put an even pull on both sides of the box. Sleds are great at exposing their weakest links. Post some more pictures and maybe I can offer some more suggestions. Rob
 
Differential is not welded. I think one side is pulling more than the other.
Box just slides on rails, no bearings.
 
Could the weight box be binding on the rails?

A few observations from the picture, take them all with a grain of salt:

1. The upper idler pulleys are well below the rails and the cable attachments on the weight box. As the box moves up the rails the cables will pull the box down harder and harder against the rails. I would either raise the idler rollers or raise the winch axle until the cables run parallel to the rails.

2. One side cable could be carrying most of the load (it looks like right side cable is sagging more than the left side cable). Is the differential locked solid in the winch axle? If it isn't a solid axle it could exaggerate any twist and binding of the weight box. A turn buckle on one or both cables could be added to even the tension on the cables. Would a double tree pulling on the center of the weight box accomplish the same thing if the frame twists?

3. It looks like the front panel is the only thing keeping that frame and rails from twisting side to side.

4. Rollers under the weight box would reduce friction and potential binding compared to sliding metal on metal. If you do this you may also need to add cushions or shock absorbers to the rear stops on the weight box.

5. The 150 degrees of cable wrap around the idler rollers could add to their deflection under heavy loads. Again, I would try to eliminate those rollers by raising the winch axle.

6. Is there enough weight on the rear of the sled to keep the tires from skidding when the weight box is near the front?
 
you said the axle has been shortened and welded, but by whom as that may be the first place to look. cant just weld an axle.
 
I think that is what we have learned. Now I'm trying to figure out if there is a way to fix it right (get a professional welder/machinist involved) or if we need to track down a different system for the front lift.
 

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