TECH5 is a regular feature where EngineLabs asks industry leaders five technical questions. This week’s guests are Mike Holzapfel, President, and Chris Raschke, Director of Sales and Marketing, at ARP Inc.
Engine Labs: How did you discover that the maximum stress in a bolt occurs in the last engaged thread, and what is your strategy to address this dynamic?
ARP: We know the maximum stress in a bolt occurs in the last engaged threads because when we perform our tensile test (pull them apart), they will always break within the first three threads, that is typically the smallest diameter of the fastener. Our strategy is to strengthen the threads by rolling them on the parts after heat treatment. This work hardens the threads and also increases fatigue life dramatically.
Engine Labs: “Tool steel” is a big buzzword in the industry now for engine components, especially in the valvetrain where stresses are incredibly high. Why aren’t more fasteners made from tool steel?
ARP: Tool steel has been around for a long time. What happens with tool steel is when you get up around the Rc50 range, the material tends to suffer from hydrogen embrittlement. If any rust starts on tool steel in this hardness range, it will allow hydrogen to penetrate into the bolt. Once hydrogen gets in the bolt and the bolt is preloaded (or tightened), the hydrogen will expand inside the bolt causing it to break. If you’re not careful with how you handle fasteners made with this material, you’re asking for problems. The good thing about tool steel is you can make bolts or studs to a really high strength so they’ll hold a lot of load and fatigue well.
Engine Labs: What are the factors in determining the lifecycle of head bolts and connecting rod bolts?
ARP: In the case of a head bolt, we determine the forces being put against the bolt. We figure out the torque that will use 75 percent of the bolts elastic limit (yield), so as long as you use the torque that is provided, the bolt will last indefinitely. When designing a connecting rod bolt, we first need to determine the load that it needs to carry. Then we need to figure out the reciprocating weight and how much rpm will be applied to the application. This is simply force equals mass times acceleration.
For example, if the force being put on the cap of the connecting rod is 10,000 lbs., we need to design a bolt that will pre load above this, say 15,000 lbs. Now the bolt won’t feel the stress of the 10,000 lb. load. Unlike the head bolt, the connecting rod bolt has more going on. The reciprocating load causes bending of the bolted joint. This situation causes bending stresses in the bolt and the rod itself. To survive this fatigue, we design rod bolts and process them differently than other bolts. For example, by engaging the thread properly and reducing the diameter between the thread and the shank diameter, it spreads the load along the bolt and offers more flexibility to accept the bending of the application.
Also, fillet rolling, putting pressure on the radius under the head of the bolt, squeezes the micro grains of the material to work harden this area, and as with all of our bolts thread rolling after heat treat increases fatigue life significantly. All of this along with fatigue testing every batch assures that the rod bolt will live beyond expectations. If you measure your rod bolts prior to installation, log the original length and the length did not grow, the bolt hasn’t yielded and is still good. This is shown on page 29 of our catalog.”
Engine Labs: Discuss pros and cons of ARP’s different hardware finishes.
ARP: “For stainless, the pros include holding its finish with no maintenance, it will not chip or crack like chrome, it’s in the 170 to 180 psi range, which is very strong, and it’s polished, so it looks great! The only con is that it can have galling in the threads, but simply using ARP Ultra Torque or anti seize will prevent this. The black oxide finish will help prevent rust to a certain extent. It may start to fade after a while and may rust in certain environments.
Cad plating is another good option, we use it on many of our wheel stud applications, especially where they may be in a lot of water, dirt and mud. It prevents rust for a long period of time and has a nice look to it. The problem with cad plating is it has become quite expensive. Dry lube is a baked on lube. It is good for applications where you do not want to apply lubricant, because you don’t want grease or oil to get into other surrounding components. It is also good for when there is no time to put lubricant on the threads, for example on NASCAR wheel studs.”
ARP: When designing flex plate and flywheel bolts, we look at the application on a high performance level and think about the maximum horsepower of the application. We select a material that will hold the preload beyond the load that is put on the flex plate or flywheel. We also evaluate clearance issues, taking into consideration how tall the head of the bolt should be as well as how much thread length can protrude through the crankshaft.