THE DEFINITIVE TRUCKING SITE

May 2008

It’s unusual to see a torque wrench being used for wheel fastening – normal equipment is a wheel-brace in conjunction with an extension pipe that stretches the studs and breaks the nuts.

Dave Scott, FleetWatch technical correspondent, puts forward a fascinating insight into what are probably the two most 'taken for granted' components that hold a truck together - the nut and the bolt. Read and study this article. It will change the way you view things.

It's fatal to feel your way into fastening - especially if safety critical components are involved. No two people will ever have the same 'feel' for fastening and even an individual will suffer from inconsistencies arising from fatigue, lubricated surfaces, tool performance variables and slight differences among fastener properties. Trailer couplings are as safe as their fasteners and in Africa, we are far too casual about specifying the correct bolt, nut and torque for safety critical tasks. Bolts and nuts are one area where brand consciousness is low, while suppliers of cheap fasteners appear to get away with sub-standard quality. Jake Venter's excellent DIY column in the 2003 February issue of Car magazine summed it up - "Bolts can drive you nuts!"

Preload
A misunderstood part of bolting items together is preload - the tension placed in the bolt by the nut (as opposed to the load). A sufficiently high preload will protect the bolt from fatigue as the load changes, as the varying load will change the clamping force on the bolted components, rather than the tension on the bolt. As a rule of thumb, the preload should exceed maximum load by around 15%. 
When fastening, there are three main ways to control the amount of preload developed. Firstly, by measuring the tightening torque; secondly the strain through measuring fastener elongation; and finally by measuring the stress in the fastener. 

Bolt torque 
In many applications, a bolt is deemed to be tight enough when it has been tightened to a specified torque. Both the SA motor and trailer industry go to great lengths to publish torque values for all important applications.

However, it is possible for a joint to fail even if it has been tightened to the correct torque. This happens because the clamping load - the force transmitted by the bolt to the joint - is more important than the actual torque reading, and in practice it's very difficult to know exactly what load the bolt is carrying. There is no direct relation between the tightening torque and the stress in the bolt, which is an indication of the clamping load. But common practice and a reckless insurance mentality of over-torque rules - especially in the case of trailers that are regarded as 'low-tech' transport equipment. 

As nuts and bolts are not perfectly rigid but stretch slightly under load, the distribution of stress on the threads is not uniform. On a theoretically infinitely long bolt, the first thread takes a third of the load, the first three threads take three-quarters of the load and the first six threads take essentially the whole load. Beyond the first six threads, the remaining threads are under essentially no load at all. This table shows the load on the threads

Therefore, a nut or bolt with six threads acts very much like an infinitely long nut or bolt - also it's a lot cheaper. There is little point in having more than six threads in anything. Nuts with National Coarse threads typically have five threads in them whereas nuts with National Fine threads have about eight threads. Nuts are usually stronger than the bolts they are on, which is to say that the bolt will usually break before the nut strips. 

Bolts enjoy a partnership with nuts of matching tensile strength; and don't underestimate the role of the washer! The accompanying chart shows a nut/bolt mating table: 

It's an advantage to work clean & slightly oiled
Experiments have shown that about 50% of the tightening torque is used to overcome friction at the bearing face of the nut and a further 40% is used to overcome friction between the mating threads. This leaves only 10% to increase the axial load in the shank which is directly related to the clamping load, and hence the stress. 

These are average values using clean components so that one can easily have a situation where the frictional resistance is so high that the shank does not get stressed at all. This happens, for example, when the components are dirty but it could also happen when the bolt is used without the designed washer, thus allowing the base of the head to dig into any softer material, such as aluminium. The point is that during assembly operations, how often does one see the bolts and nuts being retrieved from a dirty workshop floor - worse still with sawdust floating around or even heavily rusted - to interfere with torque readings?

Failure analysis expert Patrick Swan points out that "good quality fasteners are usually treated against corrosion and this oil film reduces friction during tightening. Solid film lubricant coatings greatly assist controlling the torque/tension relationship of fastener installation. But if not coated, then a slight oil film on the bolt threads will reduce frictional resistance during tightening and provide a more accurate torque reading." He adds: "A screw that is tightened in an over-greased or excessively oiled tapped-hole will create sufficient hydraulic pressure to crack the block around the tapped-hole because oil is not compressible."

Fasteners for trailer couplings - a standards example
A Jost JSK 37C 5th wheel is designed to take an imposed load of 20 tons and operate at 65 tons gross combination mass (GCM). We all know that these limits are often exceeded and it just doesn't pay to use inferior nuts, bolts and washers with such massive forces at play. Here are a few key 5th wheel standards to observe for fastening - 

•  As safety critical items, 5th wheel mountings must only use high tensile bolts to minimum grade 8.8 with a 1.5 mm thread pitch (M16 x 1.5-8.8). Obviously, where nuts are used in the 5th wheel mounting, nuts of compatible tensile strength must be used (grade 8).

• When mounting fifth wheels it is important to use correct hardened flat washers - M17 washers to DIN specification 7349. Using standard black cut washers allows the washer to collapse in the elongated hole in a 5th pedestal resulting in loose bolts securing the 5th wheel.

• It has become common practice to use 'nylock nuts' in mounting 5th wheels to base plates as well as in mounting the base to chassis side rails. This is a good practice, providing that bolts of sufficient length are used - at least 3 full threads of the bolt must protrude through the nut when the correct torque is reached.

• After three applications of a nylock nut being removed or remounted to a bolt it loses locking characteristics and should be replaced. A nylock nut must be replaced in every instance where heat has been applied to assist in loosening a nut/bolt.

• It is highly recommended to weld stop-blocks, both fore and aft of a fifth wheel pedestal, to counter the shear forces on the bolt should bolts become loose. Stop-blocks may be omitted if it can be assured that the correct torque of the bolts and consequently adequate friction contact can be maintained at all times. Paint thickness should not exceed 50 um to guarantee the fit to be friction tight

• A Jost flanged type king pin, a 5th wheel's partner, is fitted with self locking bolts and should torqued to 190 Nm (+- 10Nm).

Torque wrench technology advances
A torque wrench is a tool used to precisely set the torque of a fastening such as a nut or bolt. It is usually in the form of a socket wrench with special internal mechanisms and is used where the tightness of screws and bolts is crucial. It allows the operator to measure the torque (rotational force) applied to the bolt so it can be matched to the specifications, permitting proper tension and loading of all parts. 

A torque wrench indirectly measures bolt tension. The technique suffers from inaccuracy due to inconsistent friction between the fastener and its mating hole. Measuring bolt tension (bolt stretch) is more accurate but most often torque is the only means of measurement possible. Now we have much more sophisticated wrenches which sense not only the amount of torque but the degree of stretch so that torque force is stopped just below the yield point. But there are a great variety of torque wrenches for different tasks while torque wrench care and calibration is a discipline in itself. Here are the most common types - 

• Beam type 

• Deflecting beam 

• Click type - most popular automotive tool

• "No-hub" wrench 

• Electronic torque wrenches 

• Programmable electronic torque/angle wrenches 

• Mechatronic torque wrenches 

MISSING NUTS This wheel rim pic taken of a loaded working rig is symptomatic of SA road transport standards among many operators. Missing lug nuts and heavy rust – it’s no wonder the wheels and everything else fall off.

'Maak vas' with QSTT!
There are info-megabytes floating on the Internet around bolts-'n-nuts. Yet our streets and workshops are littered with failures and lack of fastener standards. What would be a simple and effective approach for a fleet owner who has little time on his hands to control this issue? QSTT - Quality, Safety, Torque and Training - seems to be the short answer.

• Quality - there is no substitute for quality. It extends to standards for purchasing, product-marked traceability and accountability for supply, the right nut and washer for the right bolt, storage, distribution, handling, and everything that goes with record keeping and failure analysis. There must a declared bolts-'n-nuts policy.
    

• Safety - it's just not negotiable. All safety critical components must have minimum standards for fastener replacement and detailed records kept.
   

• Torque - the right torque wrench for the right job, properly calibrated, stored and handled with care. There have to be torque charts and someone who has to record, monitor and be in charge of torque as part of a job description. It's just not a casual task keeping up to date with changes and varied requirements. Wheel rims, for example, require a range of fastener torque settings that go unmeasured and uncontrolled.
   

• Training - it's far beyond letting people tighten fasteners so that nothing falls off but everything stretches and breaks. Even being in charge of bolts-'n-nuts can be something one is proud of and well-trained in. For a start, just work clean and scrap the re-use of stretched, fatigued fasteners

Somebody's life will today depend on how a bolts-'n-nuts are mated - maak vas! But just do it properly.

References & Acknowledgements:
Jakes Venter - Car Magazine
Patrick Swan
http://mdmetric.com/tech/u19e2.htm
www.gizmology.net/nutsbolts.htm
Jost Transport Equipment