As many operators of miniature golf courses know providing a good atmosphere for your customers is essential for good business. Much of this atmosphere is provided by structures created for their novelty. At Champions Fun Center one such structure, a water wheel, has been a source of problems since the facility was opened. The purpose of this study is to determine what modificatoins could be made to the wheel and axle design to increase the life of this part.

The axle assembly has traditionally been fabricated from a two inch thick, sixty-one inch long cold finished steel bar (AISI-SAE 1018). Four .375in. thick 11.25x8.5in. steel plates (AISI-SAE 1018) are welded to the bar twelve inches from one end. The plates are separated from each other by ninety degrees and are positioned with one of the 11.25in. sides against the bar. The assembly is slid through the center of the water wheel and secured to it with the steel plates. The axle is held in position by two sealed bearing units, one at each end. One bearing is supported by the foundation of a small building. Because this bearing is mounted inside the building it will now be referred to as the inside bearing, for convenience. The other bearing is supported by a concrete piling outside the building. This bearing will now be referred to as the outside bearing. Because of the orientation of the wheel the inside bearing is significantly farther from the center of gravity of the wheel than the outside bearing is.

The weight of the wheel is estimated at 2000lbs. (wet). Because of the distance between the wheel and the bearings there are great amounts of stress on the axle near the wheel. This stress causes the axle to bend downwards. When the wheel rotates the axle is fatigued by this bending. Eventually this fatigue causes the axle to fail. The above mentioned design has been used for three years, and has failed three times. The average life of an axle has been three to four months, but because of the high cost, and labor intensity of replacing one such axle we are limited to one repair per year, traditionally at the beginning of summer.

The following options were considered for strengthening the axle:

• The use of 304 stainless steel to fabricate an axle of the same diameter as the original.
• Installation of a third bearing and support, near the inside of the water wheel.
• The use of a 2.5-inch oversized axle.

The use of stainless steel has been discounted because the cost of fabricating a stainless steel axle assembly would be four to five times higher than that of an equivalent assembly made of 1018 carbon steel. Also the use of a third support was discounted, because of cost, and the "down time" that would be required to properly install it. Thus the use of an oversized 1018 carbon steel bar in the fabrication of a new axle seemed to be the best option. To compare the service life of this new axle to that of the current one the following process was used, for each axle.

• Using the following free body diagram the reactions at each bearing were determined.

• Using the reactions at each bearing the moment of bending was determined for every inch along the axle. This moment was greatest where the axle mounted to the wheel on the side nearest the inside bearing.
• The stress on the metal was then determined from this moment of bending.
• A graph of stress vs. expected fatigue life was used to determine the expected service life of each assembly.

• The current design was determined to have a maximum moment of bending of 26891 inch pounds, and stress of 34.2 kpsi.
• The new design was determined to have a moment of bending of 27440 inch pounds, but because of the increased diameter of the bar the stress was only 17.9 kpsi.
• According the graph of an S-N curve for 1018 carbon steel (Collins 185) the current axle would fail at approximately 2 million cycles or four months of service.
• According to the same graph (Collins 185) the new axle has an infinite fatigue limit, and should last forever.

The use of an oversized bar will greatly increase the life of the axle assembly.

Collins, J. The Failure of Materials in Mechanical Design. New York: Wiley and Sons, 1981.