case studies

Snowblower Drive Attachment

Dr. Frank M. Kelso

Mechanical Engineering Department

University of Minnesota

Background

Almost all tractors have a power take-off (PTO) of some sort. PTO's are used to supply power to farm equipment (mowers, rakes, balers, and such) attached to the tractor. The PTO on the tractor shown below (right) has a splined stub shaft, to which a long green PTO shaft is attached.The green shaft is supposed to power the (rusty red) snowblower (left) but as you can see, the PTO shaft is not yet attached to the snowblower.In fact, the bolts that attach the shaft to the snowblower were sheared off, and I'm in the process here of putting in new bolts and reattaching the shaft.

tractor power takeoff

Figure 1: Snowblower (left) and Tractor PTO (right)

The dark green PTO shaft has a universal joint (U-Joint) at each end, but only the UJoint at the far left end is visible (the right UJoint, where it attaches to the tractor, is shielded by the green half-sphere as shown). I left a hammer on the snowblower so you can get an idea of the size of these components. A closeup of that UJoint is shown below.

UJoint and Flange

Figure 2: PTO UJoint and Flange

The two new bolts inserted into the flange will find a home in the mating flange of the snowblower, shown below.

receiving flange

Figure 3: Snowblower Flange

The bolts clamp the right flange (Figure 2) to the snowblower flange (Figure 3) and as the snowblower flange turns, it turns the blades of the snowblower.


Bolt Preload

The bolts are tightened almost to their full proof load.The reason for this is explained in Section 10.7 of the book. For a joint like the one shown above, I want to clamp the two flanges together as tight as possible, and let friction carry most (or all) of the load. If the flanges are only loosely clamped, then under load they can shift with respect to one another and the bolts will be loaded in direct shear. If I clamp the joint up tight, then friction will keep the flanges from shifting and the bolts won't feel much if any shear load.

Sample Problem 10.3 (p.379) illustrates the general idea. A Grade 5 bolt is tightened to its full proof load, and that pinches the joint together. Then, when a load is applied, its not the bolt that carries the whole load - it's the friction between the two surfaces. The higher the preload, the greater the frictional force that can be developed to carry the load.


Question

The bolts I picked up at the hardware store were 1/2-13 UNC Grade 5 bolts. I used two of them to clamp the flange together, and when I tightened them I went right up to the full proof load. The steel flanges are both 3 inch OD and 1 inch ID (1/2 inch thickness each). The tractor is supplying power at 1000 RPM.

How much horsepower can be transmitted before I shear the bolts again?


last modified: Monday, August 4th, 1997   /fmk/