Fastener fatigue of an engine stud on snowmobile lines supporting 700-cc reciprocating engines at Polaris Snowmobiles, Minneapolis, required a fastening system that could handle high vibrations and temperature extremes. In addition, the engines were stacked so the stresses were higher.
With snowmobiles, there are temperature extremes. When clamping aluminum with steel fasteners at 40°F below zero, which are typical operating conditions, the joint is very loose compared to what it is at operating temperatures which can be as high as 200°F. This is especially true with the long clamp length used in this location.
The company turned to Unbrako Engineered Fasteners, Cleveland, for a solution. “We looked at the current design of the engine mount and determined we didn’t have to change the design at all,” says Jon Strain, Unbrako EF’s manager of engineered fasteners. But the stud was a different matter. “We used basically the same chemical composition of the material as the competitor, but to higher standards. Then we used a proprietary combination of heat treating, thread rolling, and forming.” The fasteners also have a unique threadform, fatigue-resistant ground-fillet design, and corrosion- resistant coating.
After the studs were delivered, Polaris sent them to an independent lab for fatigue testing. The SPS fastener outlasted the competitors fastener by at least 500%. “There was just a phenomenal amount of difference between the lives of the two studs,” said Polaris’ Steve Weinzierl.
After the independent lab testing, Polaris did some testing of their own. Polaris had originally dictated that the part had to withstand 55 million cycles under operating conditions. They got some samples of the original fastener and the best test result they could get was 2.2 million cycles at the specified load, cycle rate, and rate of change. Then they tried the SPS engine stud. Polaris put them in the engine and ran them to the specified 55 million cycles. After that was accomplished, they installed them into another engine. The stud finally failed around 13 million cycles. The warranty savings on the engine stud alone was close to $8.2 million.
“I’ve been involved in a lot of front designs that don’t have flaws, where stock bolts or studs do the trick. But sometimes, as in this case, it makes sense to spend a little more on the fasteners to make the product better,” says Weinzierl.
Another company, Adams Elevator, Niles, Ohio, was looking for a solution for hydraulic elevator failure due to a new back-up mechanicalbrake system. The bolt needed to hold together a two section, 360°, mechanical brake clamp on the cylinder lift arm that activates in case of hydraulic pressure loss. For the solution Adams turned to Unbrako EF for a shoulder bolt. The device incorporates four shoulder bolts that required very high tensile strength and a high degree of concentricity. The brake engages when hydraulic pressure in the elevator drops. Plates fall to engage the plunger. It actually grabs a piece of steel that is ground to a 20 micron finish. It puts upwards of 4,000 to 5,000 psi into the plunger which causes an elastic deformation of the plunger. All of that pressure goes instantly to the shoulder bolts. There are also side rails or buttresses that the shoulder bolts pivot against. It’s a part of the device where the shear tension is created by the side plates relative to the moving arms.
When it’s applied, there’s upwards of 50,000 to 60,000 lb of shear load on this bolt, and the bolt maintains concentricity under this stress.
“To determine stress measurements we used a prototype of the device. We had finite-element analysis done on the shoulder bolts where the compressive tension and shear forces were calculated,” says Koshak. “We were able to determine what the correct shear stress had to be. After we ran out of prototype bolts, we used bolts from another product. We had to scrap a lot of them because they were not concentric. It wasn’t a shear issue, it was a roundness issue.” With over 400 installations of the device, there have been only three failures.
Microscopic photography for fasteners
The Olympus DP10 high-magnification photographic system from Unbrako Engineered Fasteners, Cleveland, part of SPS Technologies’ Industrial Fasteners Group, provides imaging capability at 1.41 million pixels CCD for fastener failure analysis. It has directprint capability and can store images electronically. The system is said to expand the focus area of a microscopic image <10 times over that of traditional Polaroid photography.