Vehicle and trailer parts

The automobile made the repair welder’s fortune. Dunham’s manual of 1917 was written for garages and repair shops welding broken car frames, cylinders, crank cases, axle housings, drive shafts and worn parts of every kind, and its author argued that a welder who could weld every part of a car could weld anything. Campbell’s Army manual of 1919 discussed welding crankshafts and leaf springs. Much of that work is now done differently, or not at all, because vehicle parts carry people at speed, and their failure on the road can kill.

This chapter looks at what the early writers said about springs, axles and crankshafts and why modern practice has changed, calculates why a leaf spring cannot be welded back to its working strength, works through the repairer’s sheared trailer axle stub, and sets out the rules that govern vehicle and trailer repairs.

The rules come first

Vehicles and trailers used on public roads are subject to vehicle standards and to the rules of the registration authorities, which set what may be modified or repaired and how, and in many cases require repairs to safety-related parts to follow the vehicle maker’s instructions or be certified. As an example of how such rules are written, New Zealand’s vehicle inspection requirements for light vehicle repairs list, as a reason for rejection, a steering or suspension component that has been welded or heated as part of a repair, unless the manufacturer’s instructions specifically permit it. Australian vehicle standards and state and territory regulators likewise restrict welding and heating of steering and suspension components. A repairer must know and follow the rules of the jurisdiction where the vehicle is registered, and when in doubt, treat the part as one to be replaced.

Springs

Both early writers were wary of welding springs. Campbell wrote that, although springs had been welded and tested, welding them with the oxy-acetylene flame was not to be recommended. He explained why: a spring is made of steel that takes a uniform hardening, and a weld introduces metal that cannot be expected to harden the same way, so that the welded zone cannot behave as spring. He criticised welders who hammered and quenched welded leaf springs to harden them, and said spring welding would not be encouraged until a filler could be found that would come out of the weld with the same properties as the spring. Dunham agreed that springs should rarely be welded, as they are high-carbon tempered steel, said that welding a spring broken through its bolt hole was practically impossible to make hold, and allowed only an emergency repair of the end of a main leaf, with the understanding that it was not permanent.

The modern reason is the same, and the numbers make it vivid.

Figure 1Bending stress in one half of a leaf spring

σ = 6 × F × L ÷ (n × b × t²)

F, load at the spring eye

10 kN

L, half-length of the spring

0.5 m

n, b, t: leaves, width, thickness

6 leaves, 70 mm, 10 mm

1. σ = 6 × 10,000 × 0.5 ÷ (6 × 0.070 × 0.010²)
2. σ ≈ 714 MPa

A spring works at stresses of several hundred megapascals, often more, because it is made of hardened and tempered spring steel with a very high yield strength. Ordinary weld metal, and the softened or hardened zone the weld leaves in the spring steel, cannot carry those stresses in fatigue. A welded spring leaf will either bend permanently, crack in its brittle heat-affected zone, or break at the weld, often suddenly. Springs are replaced, and when one leaf of a spring has broken, the whole spring pack is usually replaced or rebuilt by a spring specialist, because the other leaves have carried the broken leaf’s share of the load and may be cracked themselves.

Crankshafts

Campbell wrote that crankshafts four inches and more in diameter could be successfully welded if correct methods were used, and that a shaft that had broken by fatigue in the cheek of a throw, with no part thrown out of line, could usually be welded and brought back into use quickly. Dunham described welding and building up crankshaft journals, with the shaft kept hot and straightened in the lathe afterwards.

Today, worn crankshaft journals are normally reground undersize to the engine maker’s undersize bearings, a precise operation on a crankshaft grinder, and crankshafts with cracks are generally replaced. Specialist crankshaft repairers do rebuild some large, valuable crankshafts by controlled welding processes followed by heat treatment, stress relief and crack testing, but that is far from a garage repair. A crankshaft is a highly stressed, fatigue-loaded part, and a crack in one is evidence that its fatigue strength has already been exceeded somewhere.

Axles and drive shafts

Dunham said that rear axles and drive shafts should be welded with the same care and judgement as crankshafts, and always put in the lathe and straightened afterwards, because a shaft out of line must break, and the weld, as the weakest place, would be blamed. That advice is still sound for the alignment, but modern axle shafts are usually induction-hardened or made of high-strength steels whose properties welding would destroy, and they are replaced. Drive shafts, propeller shafts, are tubes with welded yokes, and their repair, rebalancing and re-tubing is specialist work done to the maker’s or a qualified driveline shop’s standard.

The running case: the trailer axle stub

This is an illustration. A farm trailer came in with one stub axle sheared off at the point where it entered the square axle beam. The trailer was a single-axle tipper, rated by its maker for a load the farmer admitted he had exceeded more than once on rough paddock tracks. The fracture showed beach marks spreading from the toe of the weld that joined the stub to the beam, with a final fracture zone about a third of the section. Reading it as Chapter 2 describes, the fatigue crack had grown across about two-thirds of the section before the rest broke, so the stub had been cracking for some time, probably many loads, before it finally sheared.

The bending stress at that point is easy to estimate. If each wheel carries about 1.5 tonnes, a force of about 14.7 kN, at a distance of about 80 mm from the beam to the centre of the wheel bearings, the bending moment is about 1,180 N·m, and in a 50 mm stub, the nominal bending stress is about 32 × 1,180 ÷ (π × 0.050³) ≈ 96 MPa.

Figure 2Bending stress in the trailer stub axle at the weld toe

Static, rated load96 MPa, nominal, before stress concentration at the weld toe

Rough track, about 2.5 times static240 MPa, nominal, before stress concentration at the weld toe

Overloaded by a third, rough track320 MPa, nominal, before stress concentration at the weld toe

On a rough track, bumps can multiply the load several times, so the stub at the weld toe saw repeated stresses of a few hundred megapascals, at a stress concentration, in a weld. The fatigue failure was no surprise, and overloading had made it come sooner.

What the repairer did

The repairer did not weld a new stub onto the old beam. Trailer axles are normally supplied by axle makers as rated assemblies, with the stubs welded to the beam by a qualified procedure, and their capacity depends on that procedure. The repairer fitted a new axle assembly of a capacity suited to the trailer’s actual use, chosen from the axle maker’s range with the farmer, with new bearings and hubs. It checked the other stub, the spring hangers and the drawbar for cracks, and told the farmer, in writing, that the trailer’s rated capacity was a limit, not a suggestion.

Chassis and frames

Truck and trailer chassis are sometimes repaired by welding, but vehicle makers publish body builders’ and repairers’ guidelines for their chassis that set out where welding is and is not permitted, how cracks are to be repaired, and how reinforcements must be fitted. A common theme of such guidelines is that welding across chassis rail flanges, or close to them, is not permitted, because the flanges carry the highest bending stresses and a weld there starts fatigue cracks. Repairs typically use reinforcing plates on the webs, attached as the guideline specifies. Weld Repairs That Hold covers crack repair and reinforcement in plate in its chapters on patches and inserts and related subjects; for a vehicle chassis, the maker’s guideline governs.

Other vehicle parts

Many other vehicle and trailer parts carry safety loads: steering arms and knuckles, tie rods, suspension arms and their mountings, towbars and couplings, drawbars, brake components and wheel hubs. These are replaced with parts of the right specification, not repaired by welding, unless the maker specifically permits a repair and sets out how. Parts that do not carry safety loads, such as mudguards, toolboxes, body panels and brackets for lights, can be repaired freely, with ordinary care for the materials.

Wheels, hubs and bearings

Wheels and hubs are among the parts most often brought in with damage that seems simple to fix: a cracked wheel centre, a hub with a worn bearing seat, a stud hole elongated by a loose wheel. All are safety-critical, and all are replaced. A wheel that has been welded can crack again at the weld and come apart at speed; a hub whose bearing seat has been built up or sleeved may let the bearing move and seize. Loose wheel nuts, the commonest cause of elongated stud holes and broken studs, are a maintenance problem that the repairer can address by replacing the damaged parts and showing the owner how and when to check the nuts.

Couplings and drawbars

The coupling and drawbar transfer the whole of a trailer’s towing and braking load to the towing vehicle, often with shock on rough tracks. Couplings are rated components and are replaced, not repaired. Drawbars on farm trailers are often fabricated, and a cracked drawbar can sometimes be repaired, but the crack is a warning that the drawbar is overloaded or poorly detailed, and a repair that does not improve the detail, for example by adding a properly designed reinforcement and removing a stress raiser, will crack again. For trailers used on public roads, the drawbar and its attachment are subject to the trailer construction rules, and a repair or modification may need certification.

Telling the owner no

Vehicle and trailer repairs are where a repairer most often has to refuse a customer: the farmer who wants the spring welded because the new one is a week away, the operator who wants the chassis flange welded again because it is quicker, the contractor who wants the stub axle welded back on because the trailer is needed tomorrow. The repairer’s refusal, with the reason, written on the job card, protects the customer, other road users and the repairer. Most customers accept a clear explanation, especially when the repairer offers the quickest safe alternative, such as a temporary loan trailer, a hired part or an urgent order.

Agricultural machinery on the road

Agricultural machines that travel on roads, such as tractors, harvesters and their trailers, are subject to road rules when they do, and their steering, braking and towing parts are safety-critical whether the machine is in a paddock or on a highway. The repairer treats them as vehicle parts, follows the machine maker’s instructions, and refers anything doubtful to the maker or to a qualified engineer.

A failure: the welded flange

This is an illustration. A transport operator once asked the repairer to weld a crack that had appeared in the bottom flange of a truck’s chassis rail, near a spring hanger. An earlier repairer had already welded it once, along the crack, across the flange. The crack had come back, longer.

The repairer declined to weld it again and referred the operator to the truck maker’s repair guideline, which, like many such guidelines, prohibits welding on the chassis flanges. The rail was repaired to the guideline with a bolted web reinforcement by a workshop approved for the work, and the spring hanger’s mounting, which had loosened and was the cause of the crack, was rebuilt.

Exercise: a heavier trailer

The farmer asks whether a heavier-duty stub axle, 60 mm instead of 50 mm, would have avoided the failure. Using the cube law from Chapter 3, by how much would the nominal bending stress fall? Would that alone have been enough?

The bending stress depends on 1 ÷ d³, so increasing the diameter from 50 mm to 60 mm reduces it by a factor of (50 ÷ 60)³ ≈ 0.58, a fall of about 42%, from about 240 MPa to about 139 MPa on the rough-track figure.

That is a large improvement, but a heavier axle alone does not guarantee a long life. The weld at the stub’s root remains a stress raiser, overloading still multiplies the stress, and the axle must be part of a rated assembly with matching bearings, hubs, wheels and tyres. The right answer is an axle assembly rated for the trailer’s real duty, used within its rating, which is what the repairer fitted.