The axle load of a wheeled vehicle is the total weight felt by the roadway for all wheels connected to a given axle. Viewed another way, it is the fraction of total vehicle weight resting on a given axle. Axle load is an important design consideration in the engineering of roadways and railways, as both are designed to tolerate a maximum weight-per-axle (axle load); exceeding the maximum rated axle load will cause damage to the roadway or rail tracks.
On railways, a given section of track is designed to a maximum axle load. The maximum axle load is related to the strength of the track, which is determined by weight of rails, density of sleepers and fixtures, train speeds, amount of ballast, and strength of bridges. Because track and especially the rails are expensive, it is desirable to optimise the track for a given axle load. If the track is overloaded by trains that are too heavy, it can be destroyed in a short time. It is convenient for the steelworks that rails are made in a limited number of sizes, so that a perfect match of rail weight and axle load is rarely achieved. New rail is often reserved for heavy main line use, which releases good but lighter rail that can be cascaded for lighter duties on branch lines. The lightest rail cascaded from the lightest branch lines may have no railway use other that for structural items such as fenceposts, telegraph posts and for reinforcing concrete. Increase density of sleepers and reduce axle load can help to increase train speeds.
Light railways were built with rail weighing as little as 30 lb/yd (14.9 kg/m) but main lines used much heavier rail. On former British Rail lines the rail is mostly 90 lb/yd (44.6 kg/m) or 120 lb/yd (59.5 kg/m) .
In British Rail days most diesel locomotives were built to a maximum axle load of 19 long tons (19.3 tonnes; 21.3 short tons) so the maximum locomotive weight was 76 long tons (77.2 tonnes; 85.1 short tons) for a four-axle locomotive and 114 long tons (115.8 tonnes; 127.7 short tons) for a six-axle one. Higher axle loads are now permitted, e.g. the Class 67 locomotive is a four-axle machine weighing 90 tonnes (88.6 long tons; 99.2 short tons), giving 22.5 tonnes (22.1 long tons; 24.8 short tons) on each axle.
- rail: 50 kg/m (101 lb/yd) — (choice limited to 50 kg/m or 101 lb/yd or 60 kg/m or 121 lb/yd)
- sleepers: concrete
- sleepers per kilometre : 1666 (based on 600 mm spacings) QR standard
- max speed: 80 km/h (50 mph)
- wagon axle load: 26 t (25.6 long tons; 28.7 short tons)
- locomotive axle load: 20 t (19.7 long tons; 22.0 short tons)
- swingnose crossings at turnouts: unknown
- traffic: 8 Mt/yr
- length of line: 110 km (68 mi)
- cost of line: US$185.5 million
The new heavy-duty Fortescue Railway:
- rail: (unknown)
- sleepers: concrete
- max speed: 80 km/h (50 mph)
- wagon axle load: 40 t (39 long tons; 44 short tons) (world record)
- locomotive axle load: 40 t (39 long tons; 44 short tons)
- swingnose crossings: yes
- crossing rate: 1 in 20
- turnout speed: 80 km/h (50 mph)
- traffic: 50Mt/yr
- in 2011, proposed increased to 42 t (41 long tons; 46 short tons)
- rail 60 kg/m (121 lb/yd)
- axleload 23 t (23 long tons; 25 short tons) / speed 100 km/h (62 mph)
- axleload 25 t (25 long tons; 28 short tons) / speed 80 km/h (50 mph)
- 1830 - Rainhill Trials - 2ton 3cwt axle load (Stephenson's Rocket) - 32 km/h
- 1894 - Addis Ababa - Djibouti Railway -
- 20 kg/m rail - 12t or heavier at lower speed
- 25kg/m - ??t
- 30kg/m - 14t
- 36kg/m - 17t
- 40 kg/m - ??t ; rail being replaced in 2010
- 1894 - Lokalbahn, Bavaria - 10t initially
- 1888 - Sierra Leone Government Railway - 15 kg/m rail and lighter - 5t axle load - 20 km/h
- 1904 - Senegal - Mali - 15t axle load
- 1919 - 2-8-2, India - 12t - Class XD
- 1942 - Germany; Kriegslokomotive - 15t axle load - 80 km/h
- So long as the width of the base of the rail, or timber sleepers are in use, lighter rail can be replaced with heavier rail.
The absolute maximum axle load for railways is about 40 tonnes (39.4 long tons; 44.1 short tons), above which the crushed stones used as track ballast of the rails start to be pulverised by the vibration from passing trains.
Additional calculations must be made for bridges since a span may have to carry several locomotives and waggons at once. Heavy braking also imposes limits on the strength of piers.
An understrength bridge can limit the maximum axle load for a whole line:
The term axle load is also applicable to trucks which is complicated by the fact that trucks may have more than two wheels per axle. In this case, the axle load remains the same, but the load borne by the individual wheels is reduced by having more wheels to distribute the load.
- International Railway Journal, May 2005
- Peter Ker (2011-10-24). "Fortescue takes another step in plan to boost Pilbara railway loads". The Sydney Morning Herald. Retrieved 2012-08-06.
- Federal Bridge Gross Weight Formula
- Gross axle weight rating
- Theodore Cooper devised a loading system for railroad bridges.
- Rail profile