Bolts

By Kent

Tightening a bolt may at first seem a simple enough process. Actually, it's based upon a rather marvelous invention - the threaded screw. For the first few thousand years of our life on this planet we relied upon holding things together with our hands or binding them with bits of string or nails. The screw truly revolutionized our world as with its creation we were able to hold things together in a completely new way. Safely using this great invention isn't difficult, but to do it successfully you must know a few basic principles.

You'll find a wide variety of thread types such as those used for wood screws, sheet metal screws, and even some meant for specialized applications such as a cork screw. For most applications on a motorcycle or automobile, however, you'll encounter only one basic type, the machine screw, bolt, or nut. Even then there are many differences from one to another. The most common types are the fine thread, the coarse thread, and those types in either metric (think British or Japanese) or standard (American) sizes.

Let's assume you're installing a bolt, and that you've previously removed it and know it to be the correct size, or you've acquired it from a parts store by consulting the parts manual. The first thing to note prior to attempting to install the bolt is the direction of the threads. In nearly all applications you will be turning the screw clockwise to tighten and counterclockwise to loosen. You may have heard "righty-tighty and lefty-loosy" at some point in your life and would probably never use the phrase yourself, but it is a way to remember the correct direction of turn.

Not all applications, however, follow this rule. In some cases you'll find you need to turn the bolt in the opposite direction. You will quickly figure it out if you're doing an install, but it might not be so apparent if you're trying to remove something and it just won't come off. When this happens, try giving it a turn in the other direction just to make sure it's not one of these odd cases. In particular, be wary of any bolt that's on a wheel, axle, or other application where something is rotating. Sometimes a bolt on one side will turn in one direction, while it will turn the opposite way on the other side.

Once you've figured out which way to turn the bolt, begin installation and initial tightening by hand rather than using a tool if at all possible. You do this for two reasons. The first is that it's normally much easier to start a bolt by hand because the fingers are more nimble than any tool you'll find. The second reason is that you are very unlikely to damage anything by improper installation or incorrect parts if you're only turning the bolt by hand. It's actually quite easy to accidentally try to mix up metric, standard, fine and coarse thread types, and if you do and you force things by using a tool rather than your fingers you're quite likely to damage the threads.

Once you've started your bolt and know that it is turning freely, you can spin it on with wrench until it seats. Sometimes you can just give the wrench another turn to finish tightening, but in most cases the manufacturer of whatever you're working on has a predetermined draw they want you to achieve. This means you need to torque the bolt to a specific setting, and possibly even apply a specific lubricant to the threads before tightening.

There are a couple of reasons a bolt requires a specific torque (*See detailed explanation below). One being that at the specific torque the bolt is less likely to self loosen due to vibration, and the second is to prevent damage to either the bolt or the part that is being secured. In use, the bolt will stretch just a bit and by tightening the bolt to a specific torque you are setting the preload. This minimizes the chance the bolt will break because there is no slack between the bolt head and the item being secured.

If you're just attaching a license plate to your vehicle your installation procedures, such as getting the correct preload on the bolts may not be that important but if you're bolting a wheel onto something that's going to go down the highway at 65+ miles per hour, getting it right is a matter of safety for yourself and others. Take the time to learn, invest in a torque wrench, and do it right.

Safety Tips

Use the correct size metric wrench on metric bolts and nuts. If you don't, you're very likely to have the wrench slip and damage the head of the bolt or nut, and skin your knuckles to boot.

Use the correct size standard wrench on standard bolts and nuts. If you don't, you're very likely to have the wrench slip and damage the head of the bolt or nut, and skin your knuckles to boot.

If you don't use the correct tool for the job, you're very likely to skin your knuckles (and yes, I've done it more times than I care to admit).

Pull, don't push, on the wrench. If you push on a wrench it tends to go out of line and slip off. It's far easier to keep the wrench aligned correctly if you're pulling on it.

If you must use an open end adjustable wrench (Crescent wrench), set it on the bolt so the adjustable jaw is on the side of the direction of the turn. This puts the most force on the fixed jaw and it's less likely the adjustable jaw will spring away from the bolt head.

Don't use a wrench to tighten bolts or nuts that are meant to be finger tightened only.

Use pliers on a hex head bolt or nut only as a last resort. The jaws tend to damage the head of the bolt or nut.

Don't use "cheater bars" or pipe extenders on a wrench unless you are willing to sacrifice the tool, the nut or bolt, the machine, and your health. Using a cheater bar allows so much extra force things can break almost explosively.

Threads, especially on damaged bolts, sometimes have sharp edges and can slice your finger quite easily.

Wear safety goggles.

Wear gloves when working around sharp metal, especially in the winter when your skin is dry and easily damaged.

Use cheap tools only if you, your helper, and any bystanders, all heal quickly.

Make sure someone nearby knows what you're doing and can check on you from time to time. Accidents sometimes happen despite best intentions.

*When a bolt is tightened, it elongates.  By the same token, the bolted member is compressed.  The amount of elongation and compression of the bolt and member respectively depends on material properties and connection geometry.  Generally, it is preferred that the member is compressed far more than the bolt is elongated.  This results in the smallest proportion of an applied external load to be carried by the bolt, because as a load is applied to the connection, the preloaded bolt is stretched even further, but the member is decompressed at the same time, taking load off the bolt.  In cases of cyclic tensile loading, the load amplitude applied to the bolt is significantly reduced (often by 80% or more in a well-designed connection), resulting in less fatigue and increasing the life of the joint.  For example, a bolt might carry only 20 kilograms of a hypothetical tensile load of 100 kilograms applied to the connection, and the rest of the load would be taken by the member being bolted.

For non-permanent connections that experience fatigue loading (vibration, shock, etc) where the bolts will be reused, the preload should not exceed about 75% of the bolt's proof load (the load the bolt can take without permanently deforming).  The proof load depends on the bolt size and class.  An M5 (5mm), class 8.8 bolt has a tensile stress area of 14.2 mm^2 and a proof strength of 600 MPa, giving a proof load of 8.52 kN (about 869 kg).  That means that an M5, class 8.8 bolt can carry 869 kg without permanently deforming, and the preload should be around 6.38 kN.  For a standard zinc-plated bolt, the torque factor K is 0.2 and the tightening torque can be calculated from:

Tightening Torque = 0.2 * preload * bolt diameter

In our example, we would have:

Tightening Torque = 0.2 * 6380 N * 0.005 m = 6.38 Nm

Keep in mind that the tightening torque assumes that the bolt is engaging a thread strong enough to take the load.  If using a nut, it must be the same class as the bolt for correct connection design.  If you torque a high class bolt into an aluminum thread, you will strip the threads long before reaching the correct bolt preload.  Therefore, consult the instruction manual or an expert in the field if you are not sure how to torque connections in your application.

Detailed Explanation by Topias Nieminen

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