How does Hydraulic retarder works?

Voith VR115 retarder

As the name suggests, they work the principle of hydraulics. Primary principle being the Pascal’s law “Pressure applied to any part of a confined fluid transmits to every other part with no loss. The pressure acts with equal force on all equal areas of the confining walls and perpendicular to the walls”.

Hydraulic retarders are mounted directed on the transmission ahead of the propeller shaft and immediately after the output flange. They can also be driven separately via gears off a driveshaft .They can be actuated with a separate switch or a integrated into the brake pedal.

Hydraulic retarders use the viscous drag forces between dynamic and static vanes in a fluid-filled chamber. Here the fluid acts as a medium to achieve retardation. Fluids can be of several different types either a standard transmission (gear oil), a separate oil supply, or water.

Hydraulic retarder uses vanes attached to a transmission driveshaft between the clutch and road wheels.. The vanes are enclosed in a static chamber with small clearances to the chamber’s walls (which will also be vaned), as in an automatic transmission. When retarder is actuated, compressed air pumps the fluid into the chamber, which has stator at one side and rotor which is further connected to propeller shaft on the other side. The rotary motion of rotor accelerates the oil and creates a torque. This torque pulls the rotor via helical gear. The stator moves the oil back to the rotor. This action creates a braking torque or viscous drag and as a result the rotor and further propeller shaft decelerates. The working fluid will heat, and is usually circulated through a cooling system. The degree of retardation can be varied by adjusting the fill level of the chamber. During braking the kinetic energy of the vehicles is converted to heat energy of fluid and further they are cooled via a oil cooler through engine cooling system. Once the deceleration or braking is done, the oil returns back to the reservoir and this releases the rotor and the spring moves it back to the original position.

Hydraulic retarders are extremely quiet, often inaudible over the sound of a running engine, and are especially quiet in operation compared to engine brakes. The braking torque is as high upto 500 kW, depending on the propeller shaft speed and this is about two to three times more braking power than can be delivered by the engine brake.

Voith and ZF are two major manufacturers of hydraulic retarders. Voith is a major supplier to Indian buses. They are even mandated in certain tenders called of by state transport undertakings like KSRTC. ZF calls their hydraulic retarders as intarders but principally they are same as hydraulic retarders. ZF intarders are more popular in European trucks and since they supply along with the GB, they come as a single unit. Scania makes its own range of hydraulic retarders and fits them in complimentary to Opticruise Automated Manual transmission.

Indian buses that were and is fitted with HDR:

  • Corona Seater and Sleeper – Voith.
  • Tata LPO 1618 – Voith.
  • Volvo 9400 (both single axle and multi axle) – Voith
  • Mercedes Benz O 500 RSD 3436 and 0 500 RSD 1630 – Voith
  • New Mercedes Benz Super High Deck – Voith
  • Scania Metrolink (both single axle and multi axle) – Scania


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