Machine Part Repairs by the Numbers

About this book

Machines break and wear out, and the parts they need are often not available when they are needed. A harvester stops with a worn gearbox shaft in the middle of the season; a press’s gear loses a tooth; a cast housing cracks; a trailer axle stub snaps on a rough road. The new part may be weeks away, obsolete or very expensive, and the machine is earning nothing while it waits. Repair, by welding, machining or both, is often the answer, and done well it can last as long as the original. Done badly, it fails again, sometimes dangerously.

This book is for maintenance engineers, fitters, machinists and repair welders. It assumes a working knowledge of welding and machining and basic mechanics in SI units. It explains, with numbers, when a repair is worth making, how to read a failed part before repairing it, how the stresses in a shaft change when it is reground, built up, keyed or sleeved, how to plan weld build-up, straightening and gear tooth repairs, how to repair cast housings, which vehicle parts may and may not be repaired, and how to prove and record that a repair is sound.

Early manuals as a starting point

The repair welders of the early twentieth century did a great deal of this work. M. Keith Dunham’s Automobile Welding of 1917 described building up worn crankshaft journals, rebuilding gear teeth a drop of filler at a time, welding broken shafts and straightening them in the lathe, and the limited, emergency value of welding springs. Lorn Campbell Jr.’s 1919 manual for the United States Army’s welding schools explained the two ways crankshafts break, argued against welding leaf springs, and showed new teeth added to large cast-iron gear wheels. George W. Cravens’s 1921 treatise listed the worn journals, shafts and broken parts that railway shops and mills repaired by welding.

They were skilled and resourceful, and some of what they did would now be judged unwise, especially on parts whose failure endangers people. Each chapter takes up one of their repairs, checks it against modern understanding of stress, fatigue and metallurgy, and applies it to a modern repair shop.

The running case

This is an illustration. A regional agricultural machinery repairer services harvesters, tractors, balers, augers and trailers for farmers and contractors, and does general machine repairs for local industry. In one busy season, its workshop faces a worn harvester gearbox shaft, a broken tooth on a feed auger’s drive gear, a cracked cast-iron gearbox housing, a sheared trailer axle stub and several smaller jobs. Its workshop manager must decide, for each, whether to repair or replace, how to repair, and how to know the repair will hold.

The business, its jobs, costs and results are invented for teaching. They are not KEVOS® work. Repairs to safety-critical parts must follow the equipment maker’s instructions, the applicable standards and the regulator’s requirements, and some must not be made at all.

How to use the book

Chapter 1 decides between repair and replacement, and Chapter 2 reads the failure. Chapter 3 calculates shaft stresses, Chapter 4 builds up worn journals, Chapter 5 deals with keyways, threads and splines, and Chapter 6 with straightening and distortion. Chapter 7 covers gears and sprockets, Chapter 8 cast housings, and Chapter 9 vehicle and trailer parts. Chapter 10 sets out how to prove a repair and keep its record. Every chapter ends with an exercise and its worked answer.

Limits and professional responsibility

This book is educational. Repairs to parts whose failure could injure people, including vehicle steering, braking and suspension parts, lifting equipment, pressure equipment and structural members, are governed by the equipment maker’s instructions, vehicle standards, the applicable design and repair standards and the regulator’s requirements; many such parts must be replaced, not repaired. Welding on hardened, high-strength and high-carbon steels needs procedures developed by competent people. The numbers in this book are illustrations.

An original treatment

READOS covers weld repairs to steel plate and pressure equipment in Weld Repairs That Hold, gear geometry and strength in Gears by the Numbers, and the weldability of cast iron and steels in Weldability by the Numbers; this book applies them to machine parts and links to them rather than repeating them. The explanations, the repairer’s case, the calculations, figures and exercises are original KEVOS® writing. The historical manuals are public-domain works in the United States; their methods are summarised, checked and corrected in new words, and none of their text, tables or illustrations is reproduced.