Monday, November 17, 2008

Bolt Tightening Technique

Time and technology has changed the way many cars are put together these days, there have been many new introductions for example like plastic clips, adhesive and Velcro, replacing trim nails and screws that normally held panels together in the past. There is still the use of bolts when assembling mechanical components, but even some of these has changed. Today, on major assemblies, torque to yield bolts are used in engines.

These bolts stretch to the yield point when tightened; the bolts are often used when exact clamping loads are needed on parts, they can be used on connecting rods, crankshaft main bearing caps, cylinder heads, front engine dampers and even flywheels. Torque to yield bolts will no doubt be used if the shop manual indicates a bolt needs to be discarded and replaced with a new one.

This can become expensive and a total waste of time if you have to buy new bolts all the time. Too many these old bolts may still look usable but beware looks can deceive, if using old bolts it may cause engine failure an expense nobody wants. Through the process of tightening bolts it pulls them to their elastic limit due to stretching, that’s why new ones are needed to give maximum force on today’s engines.

It is easy to understand if you tighten a bolt to much then the harder it is for it to become accurate on a clamping load. Friction occurs when tightening bolts these are like a ramp, when turning this has to slide against another thread or ramp in the bolthole. The tighter it goes the harder it becomes to move or push up the threads ramp.

When stationary the term used is stiction as against friction, as the bolt becomes tighter it is the station that will affect a torque reading. People may use lubrication to help threads slide easier but the quality between one lubricant and another can vary. It is far easier to follow manufacturer’s guidelines for the best type of lubricant that is needed for the bolt when tightening. This is important because if you use the wrong lubricant it may become too tight or not tight enough.

There are several stages to follow when tightening torque to yield bolts this is done using both a torque and turning angle. The first step is to tighten the bolt to a low torque specification, this is done to ensure even clamping load is in place when parts are assembled. The second stage is to tighten again slightly to the higher torque stage. This will still allow enough friction and stiction on the bolt threads as the torque is tightened very little. The normal procedure after this is too turn each bolt a specified number of degrees normally two to three steps. As an example, the bolt might be turned 90 degrees, and then another 90 degrees until a further 70 degrees has tightened the bolt sufficiently. This will ensure the clamping force that has been exerted by the bolts is both accurate and even.

Special tools are used that can measure the degrees a wrench or bolt has been turned, these are low cost protractors that has a movable pointer, whilst some tools use electronics when measuring a turning angle these can cost several hundred dollars. These two types both work well and though the electronic one is more expensive it actually can make the job go faster.

Some may ask the question why are the torque to yield bolts needed now? When perhaps they weren’t needed in the 1960’s or 70’s. The answer to that question is quite simple in that engine materials are different now. For example, the aluminum cylinder heads expand differently to cast iron blocks. When an engine warms up, all the parts bolted together have to be able to slide on their gaskets or move against each other. Because of the elasticity in the torque to yield bolts allows movement between the parts but still maintains even sealing and clamping loads.

Engine parts today are a lot lighter than decades ago, the heavy cast iron parts was able to take variations in torque without failing. Aluminum alloys and thin wall castings that are used today need accurate torque if this is incorrect then leaks or warping can occur.

Today the design of engines is changing due to the need of even clamping forces, the car manufacturer Ford have prototype engines where bolts which hold the cylinder head on can go all through the engine block threading into the bottom casting which holds the crankshaft. It’s hard to imagine that only a few bolts can hold a complete engine together. This would definitely change the way engines where assembled and disassembled in the future.

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