Performance - Tractive Effort
The most important feature of the Class 60 Locomotive regarding its ability to pull heavy trains is its very high tractive effort (T.E.). This is the amount of train hauling force that can be produced by the locomotive at its coupling. In particular, the tractive effort of the locomotive at standstill governs what weight of train can be started according to the gradients over which it is run. This high tractive effort requires a wheel/rail adhesion of at least 39.5% depending on wheel diameter. As a guide to locomotive performance, the approximate traction current required to hold a train on a gradient can be calculated. This figure is important because, if the locomotive cannot achieve this value because of a fault or due to low adhesion, it is impossible to start the train. Slightly more traction current will be needed to actually move the train uphill; approximately 10% extra is usually needed to overcome static friction etc.
Achieving such a high T.E. from a 129 tonne locomotive is done by a combination of low weight transfer bogies, sophisticated traction motor control and adequate rail sanding. The low weight transfer bogies ensure that no wheelsets are offloaded when pulling hard and made more likely to slip. Actual weight transfer is zero within each bogie, and only 6% between the bogies at full T.E. Exact control of the traction motors is vital, to ensure that the maximum T.E. is applied by each wheelset according to the wheel/rail adhesion at the time. Effective sanding ensures that in poor rail conditions, the rail head adhesion can always be improved to give adequate locomotive performances. The Class 60, although designed to pull heavy freight trains, does not require any more traction horsepower to operate on 4,000 tonnes trains compared to those of 2,000 tonnes. Although the T.E. of the Class 60 is much higher than Class 56 and 58 locomotives, the increased tonnages able to be started do not require a proportional increase in traction horsepower to be worked effectively. As an approximate guide, the balancing speed a locomotive can be expected to achieve on a gradient ( after the momentum of any excess speed at the bottom of the gradient has been lost ) can be calculated. The train loadings specified by the BRB for Class 60 locomotives were dependent on routes to be worked generally, and, if length limits were not exceeded, the maximum load a Class 60 could work based on the worst case of gradient and length of gradient which was to be encountered. At all points, the locomotive should be able to start the train from standstill. The T.E. required to start a train on a gradient as previously described, was calculated assuming that around 30% ( instead of the full 40% ) rail adhesion is available. This gives allowance for bad rail conditions and allows for excess T.E. to be available for moving the train off.
The cab ammeter is not marked to show the maximum allowable traction current and, unlike some older locomotives, does not have coloured zones to show potential overheating regions. The locomotive control system works out when its traction motors are getting too hot and takes action. The locomotive performance in slow speed operation is unchanged with regard to train hauling ability.
