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What is Torque Converter Stall Speed & Why Does It Matter?

Gear heads and gear engineers both recognize stall speed as a critical figure for torque converters. In automobiles, marine drives, and high shock load industrial transmissions, torque converters (specialized fluid couplings) allow variable speed operation as well as sudden starts and stops. Prior to the invention of fluid couplings in 1905, clutches were required to take drives in and out gear. This dry shifting approach creates friction and damage over time, as gear contact surfaces wear away. Fluid couplings allow for smooth operation, as the fluid sealed within the coupling effectively absorbs torque, provides gear reduction, and also transmits power.

What is torque converter stall speed?

To answer this question, we must further consider the mechanics of a torque converter. If we could cut open a torque converter, we would see three main parts: A pump or impeller (the fan-like driving disc), which is driven by the engine; a turbine, which absorbs the motion of the fluid sent by the pump and passes it on to the output shaft; and in-between the two, a stator. You can think of the pump and the turbine as two fans facing each other. When the first fan blows, it will turn the second fan. However, if you hold the second fan still, the first fan is still able to run. The fluid inside a fluid coupling allows the turbine to stop, start, or run at slow speeds while the input shaft may still be turning at high RPMs. However, once the fluid has passed through the turbine, it must return to the pump to cycle through again. The stator alters the flow of the fluid so that it matches the pump rotation.

The stator modifies the fluid as it returns from the turbine to the pump. Within a fluid coupling, the fluid dynamics occur in the shape of a torus, or donut, with two torus shapes rotating side-by-side. Compared to traditional fluid couplings, the torque converter’s impeller and stator have curved, angled blades facing in opposite directions.

The stall speed is the maximum RPM that can be achieved while in a forward operating range without causing any motion in the drive shaft, and without causing overheating. It is a measurement of how fast the pump can run without causing problems. While torque converter manufacturers indicate a general range of stall speeds, each setup will have its own unique stall speed, depending on a number of influences, including drive torque, gear ratios, and more.

The Importance of Torque Converter Stall Speed
Stall speed is the input speed at which there is 100% slip, meaning that the engine is rotating the pump but it is not creating enough torque to turn the turbine. Torque converter stall speed describes the top RPM at which the pump may run without causing misalignment and other damage. If the pump is running too fast, it can cause overheating and eventually failure.

Call Mar-Dustrial for advice on how to incorporate torque converter stall speed considerations when replacing or servicing a Falk coupling. We can help you choose the best coupling for your application, drawing on our decades of experience in selling and repairing industrial drives.

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