A torque converter is a fluid coupling used in many heavy construction machines to transfer power from the engine to the transmission. It lets a loader, bulldozer, excavator, or haul truck start moving smoothly without a manual clutch. Its most important job is to multiply engine torque when the machine is starting, pushing, digging, or climbing.
This matters because construction equipment often needs strong low-speed pulling force more than high road speed.
Inside the torque converter, the engine spins the impeller, also called the pump, which throws transmission fluid outward. The moving fluid strikes the turbine, causing it to turn the transmission input shaft. A stator redirects returning fluid so it helps the impeller instead of fighting it, which is how torque multiplication occurs at low speed.
As turbine speed gets closer to impeller speed, multiplication decreases and the converter acts more like a simple fluid coupling.
Key Facts
- Torque = force x radius, so a larger turning force or larger radius produces more twisting effect.
- Power = torque x angular speed, written as P = τω.
- A torque converter uses transmission fluid to transfer energy from the impeller to the turbine.
- Torque multiplication is greatest when the turbine is moving slowly and the impeller is spinning faster.
- The stator redirects fluid flow to increase torque at low output speed.
- Slip = impeller speed - turbine speed, and some slip is normal in a non-locked torque converter.
Vocabulary
- Torque converter
- A fluid power-transfer device that connects an engine to a transmission and can multiply torque at low speed.
- Impeller
- The engine-driven pump inside a torque converter that accelerates transmission fluid outward.
- Turbine
- The driven wheel inside a torque converter that receives energy from moving fluid and turns the transmission input shaft.
- Stator
- A stationary or one-way-clutched blade assembly that redirects fluid returning from the turbine to improve torque multiplication.
- Slip
- The speed difference between the impeller and turbine that allows smooth starting but also creates heat.
Common Mistakes to Avoid
- Thinking a torque converter is just a clutch, because it does not usually make direct friction contact between the engine and transmission during normal fluid operation.
- Ignoring the stator, because the stator is the part that redirects fluid and makes torque multiplication possible at low speed.
- Assuming torque multiplication happens at all speeds, because it is strongest when the turbine is slow and becomes much smaller as turbine speed approaches impeller speed.
- Forgetting that slip makes heat, because the energy lost during fluid motion must be removed by the transmission cooling system.
Practice Questions
- 1 A torque converter impeller is spinning at 1800 rpm and the turbine is spinning at 1200 rpm. What is the slip in rpm?
- 2 A machine engine sends 600 N m of torque into a torque converter. If the converter has a torque multiplication ratio of 2.1 during startup, what output torque reaches the turbine?
- 3 A loader is pushing into a pile of gravel and moving very slowly. Explain why the torque converter can help the machine produce strong pushing force without stalling the engine.