Stall torque is the torque of a device at zero rotational speed or the torque load required to bring it to zero. Electric motors, steam engines, and hydrodynamic drives produce stall torque. Torque converters multiply stall torque and have ratios ranging from 1.8:1 to 2.5:1. The formula relating torque, stall torque, and RPM is T=Ts-(NTs/Nf).
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 required to bring the rotational speed of the device down to zero. An engine stall torque is equal to the engine’s peak torque, which is usually zero rpm.
Devices that produce this type of torque include electric motors, steam engines, and hydrodynamic drives. For electric motors, torque can still be measured even when these motors are stalled. Maximum continuous stall torque refers to the maximum amount of torque a stalled motor can deliver without overheating or damage.
Hydrodynamic devices such as a fluid coupling, a device used to move mechanical power from one pass to another, can also produce stall torque. The torque of this device is the maximum amount of torque it can produce without breaking or damaging itself. Measuring torque while the unit is stalled is the best way to gather accurate information.
Various types of torque converters also have measurable stall torques. 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 General Motors fluid dynamics engineer. The Borg Warner converter is considered the standard for its unique qualities when stalled, including its resistance to moving into stall mode.
A torque converter multiplies the stall torque ratio of a normal torque device. It is used to propel vehicles with automatic transmissions by revving the engine without being tied to the transmission. Torque multiplication by a converter depends on 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 the revolutions per minute (rpm) of a turning device. At zero torque, the maximum speed can be reached. Peak RPM occurs when the engine is not under any specific load, such as an automobile chassis or large conveyor. In this case, the speed is indicated as the free speed. The formula that relates these different factors is: T = Ts – (N Ts ÷ Nf) where T is equal to the torque at N rpm, Ts is equal to the stall torque and Nf is equal to the free rpm.
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