Locomotive Air Compressor Parts: Wear and Sourcing

The air compressor is one of the hardest-working machines on a locomotive. It supplies air for the train brakes and often for sanders, horns, wipers and control equipment as well. When it falters, the effects spread quickly: slow brake release, extended pump-up times at the depot, and moisture and oil in an air system that is supposed to be clean and dry.

This article looks at the compressor parts that wear, the symptoms that point to trouble, and how to keep the right spares available.

A quick look at locomotive compressors

Many locomotives use reciprocating piston compressors, often with two stages and an intercooler between them. On diesel-electric locomotives the compressor may be driven directly from the engine or by its own electric motor; electric locomotives use motor-driven compressors. Some newer units use rotary screw compressors instead. The parts list differs between types, but the wear principles are similar.

Compressor parts that wear

Valves

Suction and discharge valves open and close with every stroke. Valve plates, springs and seats wear, crack or carbon up over time, and a leaking valve reduces output and raises temperatures. Valves are among the most frequently replaced compressor parts.

Piston rings and cylinders

Rings wear against the cylinder bore. Worn rings let oil past into the compressed air and reduce efficiency. Cylinder bores wear too, and may need honing or replacement at overhaul.

Bearings and bushes

Crankshaft and connecting rod bearings, and small-end bushes, wear with running hours. Poor lubrication or contaminated oil speeds this up considerably.

Gaskets and seals

Head gaskets, valve cover gaskets and shaft seals harden and leak with heat cycling. They are inexpensive but are often the cause of oil leaks and lost pressure.

Intercooler and aftercooler

Coolers can block externally with dirt and internally with oil and carbon, and tubes can leak. A poorly performing cooler raises discharge temperatures and pushes more moisture downstream.

Filters and unloaders

Intake filters need regular replacement, particularly in dusty conditions. Unloader valves and governors control when the compressor works, and their seals and diaphragms wear like other valve parts.

Warning signs to watch for

Operators and maintainers often notice compressor problems before a failure. Typical symptoms include:

• Longer pump-up times to reach main reservoir pressure.
• Oil in the air system, found at drain valves, the air dryer or downstream equipment.
• Excess moisture in reservoirs or brake equipment.
• High discharge temperatures or discoloured discharge pipework.
• Unusual noises, such as knocking from bearings or clattering from valves.
• Rising oil consumption.

Each of these is worth investigating early. A compressor that is pushing oil and water into the air system will eventually cause trouble in the brake valves, which are more expensive and harder to source. Our brake parts buyer's guide explains how the brake system depends on clean, dry air.

Planning compressor spares

A practical spares plan for compressors usually includes:

1. Routine items on the shelf: filters, gaskets, valve kits and seals in line with your maintenance plan.
2. Overhaul kits timed to planned overhauls, ordered early enough to arrive before the unit comes in.
3. Major components such as crankshafts, cylinders, heads and coolers, assessed on lead time and the consequences of a long wait. For fleets with several units of the same type, a rotable exchange compressor can keep downtime short.

Lead time is the variable that most often catches buyers out, especially for parts sourced from overseas manufacturers. See our guide on planning imported rail spares.

Overhaul or exchange?

When a compressor is due for overhaul, there are two common approaches. The first is to overhaul it in place or in the workshop while the locomotive waits. The second is to fit an exchange unit and overhaul the removed compressor afterwards, ready for the next locomotive. Exchange keeps downtime short but needs an extra compressor in the pool. Which approach suits you depends on fleet size, workshop capacity and how quickly parts for the overhaul can be obtained.

OEM, aftermarket or made to order?

Compressor parts are available from OEMs and from aftermarket manufacturers, and the right choice depends on the part. For performance-critical parts such as valves, rings and bearings, ask about material, manufacturing method and inspection, and compare total cost rather than unit price. Our article on OEM versus aftermarket locomotive parts covers the questions to ask.

Where a part is no longer available, components such as bushes, pins, brackets, covers and some housings can often be re-engineered and machined to order. Read more in re-engineering obsolete locomotive parts.

Good practice that extends compressor life

• Change intake filters on schedule, and sooner in dusty conditions.
• Use the specified oil and keep it clean.
• Keep coolers clean externally and check them for internal fouling at overhaul.
• Drain reservoirs and maintain the air dryer, so moisture does not cycle back through the system.
• Record pump-up times, so a deteriorating compressor is noticed before it fails.

How Deomac can help

Compressor parts are part of the locomotive spares range we supply. Through our global trade service we source OEM and aftermarket compressor parts, and our workshop capabilities in CNC turning and milling cover many made-to-order components. Search our parts list or contact our Australian office with part numbers, quantities and the compressor model.