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Stall torque refers to the torque of a device when that device's rotational speed reaches zero. It can also refer to the torque load needed to cause the device's rotational speed to drop to zero. A motor stall torque is equal to the motor's maximum torque, which is usually zero revolutions per minute.
Devices that produce this sort of torque include electrical motors, steam engines, and hydrodynamic transmissions. For electrical motors, torque can still be measured even when these motors are stalled. Maximum continuous stall torque refers to the greatest amount of torque that a stalled motor can provide without overheating or causing damage to itself.
Hydrodynamic devices such as a fluid coupling, a device used to move mechanical power from one step to the next, can also produce a stall torque. This device's torque corresponds to the maximum amount of torque it can produce without breaking or damaging. Measuring torque while the unit is stalled is the best way to gather accurate information.
Various types of torque converters have measurable stall torques as well. For these devices, the stall torque is greater than or equal to the maximum output torque for a given input speed. The industry standard by which all other converters are judged is the Borg Warner torque converter, designed by a fluid dynamicist from General Motors. The Borg Warner converter is considered the standard because of its unique qualities while stalled, including its resistance to moving around in stall mode.
A torque converter multiplies the stall torque ratio of a normal torque device. It is used to move vehicles with automatic transmissions by allowing the engine to spin without being tied to the transmission. Torque multiplication from a converter is dependent upon four factors: the size and shape of the turbine, and the size and shape of the stator blades. Typical ratios for automotive purposes range from 1.8:1 to 2.5:1.
Torque is also related to a rotational device's revolutions per minute (rpm). At zero torque, the maximum rpm can be reached. Maximum rpm occurs when the motor is not under any specific load, such as an automobile frame or a large conveyor. In this case, the rpm is referred to as free rpm. The formula which relates these different factors is: T = Ts - (N Ts ÷ Nf) where T equals the torque at N rpm, Ts equals stall torque, and Nf equals the free rpm.