Power at Work: What you should know about bolts, metric and imperial

Monday, December 1, 2008

Some tips to help you through the maze of different grades and nomenclatures of bolts in use around farm machinery

by RALPH WINFIELD

Most of us buy bolts by size, but now that we have both the old imperial system and the more recent metric system, we can easily be confused. When we add the two grading systems for bolts, we become really confounded.

Bolt sizes. The old imperial system sizing was straightforward. We start with a nominal thread diameter of 1/4 inch and go up in 1/16 inch increments until we hit 3/4 inch diameter. These bolts can be obtained in a variety of lengths, usually in 1/4 inch increments from less than one inch to special order long bolts.

The one major difference with this bolt group is the thread pitch. Bolt threads can be either national coarse (NC) or national fine (NF). The NF bolts will hold torque better because of the lower pitch angle. However, with our improved bolt quality, the fine-threaded bolts are becoming less common. 

In recent years, since many of our vehicles and tractors are being built off-shore, we are seeing more and more metric-sized bolts.

Once you recognize the sizing nomenclature, you realize that it does make sense. The first number after the M will be the nominal thread diameter in millimetres. The smallest number is usually five and can go as high as 48, but usually not over 24.

The second number will be the thread pitch in millimetres and the third number will be the bolt length. Thus a typical metric bolt would be specified as M16 x 2.0 x 50 millimetres. If you see a bolt with an actual number on the head reach for a metric wrench or socket.

Imperial bolt grades. Again, let's start with the old imperial system. Bolt grades are always shown on the bolt heads. The term "hex" head refers to the most common six-sided bolt head.

If the bolt head has no marking, it will be a grade one or two. That is the lowest strength of bolts available. Grade two applies to hex cap screws (not hex bolts) up to six inches long. Grade one applies to hex cap screws over six inches long and for all other bolts and screws. Similarly, grade one or two nuts also carry no markings.

The next step up is a grade five bolt, which will have three radial lines on the bolt head. The orientation can vary, but the most common is when the three lines are equally spaced as illustrated.

A grade five nut should have one small notch at the mid-height of all six corners. Grade five bolts are superior in strength to the grade one or two bolts and will hold higher torque loads better.

Grade eight is the highest grade of bolt available in the imperial system. These bolts will have six radial lines on the bolt heads. Again, the orientation can vary but the most common layout has the six lines equally spaced, one to each point of the hex head as shown.

Metric bolt grades. The lowest grade will show the number 4.8 on the bolt head and 5 on one castellation of the corresponding nut. The second grade category can be shown as either 8.8 or 9.8. Again the numbers 8.8 or 9.8 will appear on the bolt head and the number 10 will be on one castellation of the corresponding nut.

The third metric strength grade is 10.9. The 10.9 will again appear on the bolt head with the 10 on the corresponding nut. The 10.9 is probably the most common bolt grade used on farm machinery.

The fourth or upper metric grade is 12.9. Again the 12.9 will appear on the bolt head, but the corresponding high-grade nut will have the number 12 on it. These bolts will be used where very precise high torques are required, such as attaching metal pads to the chains of track-laying vehicles.

Finding the correct torque values. Virtually every current operator's manual for a tractor or combine will show specific torque requirements for critical attachments such as wheels, rasp bars and straw chopper blades.

In addition, most operators' manuals contain a listing of guideline torque values in the specification section of the manual. These values are only to be used when specific torque values are not provided.

Many of these guideline torque values for various grades of bolts will have two columns, one for lubricated, the other for dry. Some fasteners (bolts) have phosphate or oil coatings, which put them in the lubricated category even though you did not lubricate them. The variation in required torque can often be 20 to 30 per cent lower when the fastener is lubricated. BF

Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.