It pays to pay attention to proper bolt torquing
Wednesday, November 5, 2014
Understanding the logic of bolt strain will help you realize the importance of double-checking the torque values, especially on those short bolts that are often used on many vehicle and farm equipment wheels
by RALPH WINFIELD
When I started working in the local repair garage in the 1950s, torquing wheel studs and head bolts was something you did by a guesstimated equal pull on a breaker bar. Breaker bar was (is) still a common term for a half-inch drive-socket holding bar with a swivel head, an integral part of every toolbox contents.
Torque values given in pound-feet (lb-ft) and also Newton metres (N.m) were virtually unknown in those days. Torque values still primarily quoted in imperial units (lb-ft) are the units for a pull in pounds at a distance in feet. For example, if you exert a pull of 100 pounds at a distance of one foot from a nut or stud, you are creating a torque on the stud of 100 lb-ft. If the breaker bar is 18 inches long from the pull section to the centre of the socket, a 100-pound pull will create a torque of 150 lb-ft on the stud/nut. (Please note lb-ft is correct. Though often used, ft-lb is not correct!)
Newer torque wrenches that can be set to click or flash at a preset torque setting have become required tools for every mechanic and should be available in all farm shops. Some of you may still be using the older deflection-type torque wrenches available in earlier decades. They work just fine for low-torque values. The wrench handle bends (deflects) while the pointer stays straight and points to the torque value.
Why torquing is important. The first torque values became requirements for replacing engine heads. As compression ratios became higher, especially with diesel engines, we could no longer use the old-style thick head gaskets that were somewhat forgiving to variable torquing values.
Engine heads must be torqued uniformly, and in some cases in proper stages or sequences, as specified by the engine manufacturer. When my brother and I started a diesel repair business in the late 1950s, one of our first purchases was a three-quarter-inch drive torque wrench that had a long two-piece arm and a dial gauge to show the torque. Many large engines required head bolt torques in the 300-400 lb-ft range. Torquing was a two-person operation! One of us had to read the dial while assisting with the pull.
Mechanical torque multipliers are now available for those and other high-torque applications. Most torque multipliers use the sun and planetary gear system I described in an earlier article "What you need to know about planetary gear sets," published in Better Farming, May 2014.
Uniform torque values are increasingly critical for many applications to prevent distortion and/or component loosening. One example has been our switch from drum to disc brakes on cars and pickup trucks. Drum brakes were forgiving; disc brakes are not. If the wheel studs are not properly torqued in the correct sequence, the disc rotor can be distorted, which will provide uneven braking or, worse yet, a loose wheel.
But we have another wheel factor. We have seen a significant switch from steel wheels to aluminum alloy wheels. Those wheels must be torqued uniformly.
Let me give you a notable example. Upon getting my car back from an oil change service, which didn't include tire rotation, the invoice carried the following generic statement: "Please note: If your wheels were taken off for any reason during your visit, we encourage you to come back to us in 48 hours so we can RE-TORQUE your wheels at n/c to you to ensure your SAFETY. No appointment is required!!! We thank you for your business."
At about the same time, a wheel actually came off my wife's car, which also has alloy wheels. I had torqued the wheel lug nuts after removing the winter tires and replacing the alloy wheels. Very fortunately, the incident occurred in town at a very slow speed. Damage was minimal and no one was hurt.
In recent years, many truck wheels have come loose from trucks causing serious consequences on the highway. More stringent regulations have been imposed for rechecking wheel nut torques.
It is very interesting to see that many trucking firms have adopted the plastic arrow system on truck wheel nuts. The arrows all point in one direction. Any change in an arrow's direction can be noted by the driver on his/her daily circle check of the vehicle.
Many other wheels on farm vehicles, from trailers to combines, are also of concern. Have you noticed that many manufacturers and assemblers have adopted a policy of paint-marking all nuts or studs following torquing during the assembly process?
This is an excellent practice that many of us should adopt for our own vehicles and farm equipment.
The ultimate torquing challenge. While very few of you own large swinging units, such as track-laying backhoes, the design and assembly challenge of the "Swing Drive" is worth noting. I guarantee that you will look at sequential torquing in a whole new perspective.
Many of the principles of proper sequential torquing also apply to the ring of "ramping bolts" that have been and still are used to mount some rear tractor rims. If one ramp bolt comes loose, torque will be lost on all the other ramp bolts. Immediate corrective action is required.
What creates torque? No, it is not the breaker bar or the torque wrench. It is really the stress created in the steel bolt because of the strain resulting from the thread angle. As the bolt or nut is turned, the thread angle tries to stretch the bolt. The bolt quality (grade) determines the bolt's ability not to distort but to hold the torqued value. For example, a fine-threaded grade 8 bolt will hold torque much better than a coarse-threaded grade 5 bolt.
Bolt length is also a very important torque-holding factor. The longer the bolt, the better it can hold or retain the torque you apply.
The ultimate example. If you look at the "Swing Drive Bearing Ring" on a large tracked excavator, you will see that the double ring will be mounted with 60 to 80 bolts. Half of them will be on each ring – that is, 30 to 40. But please look again and you will see that each one of the inner (lower) ring bolts will have about a two-inch (five-centimetre) length of double-walled pipe on the bolt.
That piece of pipe is not there because somebody ordered bolts that were too long. The longer bolt is absolutely essential to ensure that the bolt can maintain the strain and thus the high torque (450-500 lb-ft) that was put in during assembly.
Regardless of the number of bolts being torqued, the sequence is important. Basically, you must use a progressive opposite sequence – in other words, working from the centre of each quadrant, especially when 30 to 40 bolts are involved.
Bolt shape changes that have occurred. You will see in one picture at the bottom of this page a new "shanked bolt" that is preferred or required for many high-torque applications, such as engine head bolts and slewing rings (swing drives).
First of all, note the flanged head that provides a larger bearing surface but also a curved underside to eliminate the high stress point at the head/stem junction. But also look farther down the stem or shank, and note the smooth transition to the longer shank/stem section of the bolt.
It is that smaller diameter shank section of the bolt that is going to be stretched (elongated) during the torquing process. The strain in that section allows the bolt to maintain the torque applied.
But please look at that bolt again. Note that the threaded section is again larger than the shanked section. This reduces the strain on the threaded section, but especially at the shank-to-thread transition.
Most of you will have twisted off bolts. Where do they usually break? They break at the transition point between the shank and the threads. This is a high stress point in standard bolts that have the threads cut into a regular shank.
My example. You will see, below, a picture of the head section of a 40-year-old six-horsepower engine. Surprisingly, the head bolts for it are all of the shanked type. When I had it re-ringed to stop significant oil consumption, I tried to cheat and not follow the torquing sequence given in the owner's manual. The next time around – after one hour of operation with a new head gasket, I did follow the torquing sequence, which required removal of the fuel tank from the mounting frame to provide full access to every head bolt.
That little engine is running just fine and I am reminded that both torque values and sequence are very important. For your information, the oil consumption is now virtually non-existent!
To conclude, if you understand the logic of bolt strain, you will quickly realize the importance of double-checking the torque values, especially on those short bolts that are often used on many vehicle and farm equipment wheels.
Many farm shops will require three torque wrenches – a small quarter-inch drive unit for working on small engines, a half-inch drive unit for the car and small wagon wheels, and a three-quarter-inch drive unit if you are working on larger projects, such as rear tractor wheels or big engines.
Please be advised that after each use, the torque setting on the torque wrench must be turned back out of the working range. This action, which only takes a minute, must be taken so that the torque wrench does not lose its calibration. New units usually come with a calibration certificate from the manufacturer.
We cannot afford to use the torquing technologies of the 1950s! BF
Agricultural engineer Ralph Winfield farms at Belmont in Elgin County.