Hydrostatic transmissions (HSTs) are widely used in all types of mobile equipment. Not only are they compact and lightweight, but they allow great freedom in design by eliminating the physical constraints of drive shafts and right-angle gear drives. They also allow a wide variety of control configurations, such as speed, torque, and power limiting control, and load sensing. Furthermore, adding a second pump takes up little additional space, yet provides power and control for work functions.

Once a vehicle is moving, the challenge becomes how to stop it. HSTs also provide power for deceleration — and unlike mechanical brakes, no friction surfaces exist that need to be serviced regularly. HSTs convert the kinetic energy of a moving energy into fluid energy that can be dissipated by the hydraulic system. This hydraulic energy is instead stored in an accumulator. This configuration essentially converts braking energy into hydraulic energy, then back into mechanical energy to accelerate the vehicle. Although discussion of these hybrid hydraulic drives is beyond the scope of this article, it does demonstrate the versatility of hydrostatic drives.

However, when a machine’s prime mover is shut down, so is the HST. Without the engine and HST operating, obviously, there is no power transmitted for propulsion. But what may not be so obvious is that with the HST disabled, braking may also be disabled. This is why most mobile equipment powered by an HST incorporates a spring-applied, hydraulically released parking (locking) brake at each wheel.

These brakes prevent wheels from rotating when there is no hydraulic pressure, which makes them effective as parking brakes. Once the HST is powered up, hydraulic pressure releases the brakes so the wheels can turn. And depending on the type, travel speed and mass of the machine, the brakes can also act as a fail safe device by applying braking force to the wheels when a catastrophic loss of pressure occurs — from a broken hydraulic line or severed hydraulic hose, for example.

The right configurations for the application
Different brake configurations are used depending on the type of machine. The parking brake is usually controlled by a 3-port, 2-position (3/2-way) directional control valve that is actuated electromagnetically. Energizing the solenoid of the 3/2-way valve routes pressurized fluid from the main hydraulic system to the brake. The hydraulic pressure then actuates a brake piston to release the brake.

Cutting power to the solenoid shifts the valve back to its relaxed state, so it routes fluid from the brake’s hydraulic line to tank. With no pressure acting on the brake’s piston, spring force then actuates the brake. Likewise, if some failure in the hydraulic system causes a loss of pressure, the brake’s spring assembly will close the brake, causing the vehicle to stop. The spring assembly should be designed to apply enough force to stop the machine safely, even on an incline.

Towed in a hole
But there is one other situation: a vehicle is disabled and must be towed. With the engine not running or the hydraulic system out of commission, some means must be provided to release the parking brake.

The actuation of parking brakes is governed by DIN standard EN 500-1, “Mobile Road Construction Machinery — Safety.” The standard specifies: Mobile road construction machines with an operating weight exceeding 2000 kg must be fitted with fixtures suitable for towing (hooks, rings, eyelets), making it possible for the machines to be towed away from the danger zone across short distances (less than 300 m).” But, again, the spring-loaded brake on a disabled machine will prevent it from being towed. Three methods have been developed to release the parking brake of a disabled vehicle: mechanical release, hydraulic release, and pressure-dependant resetting.

Mechanical release
Some manufacturers of hydrostatic wheel motors with an integrated parking brake use an emergency mechanical release. This release mechanism uses a threaded spindle or an eccentric tappet to compress the spring assembly, thereby releasing each brake separately. This solution is both cost-effective and efficient.

However, it does have some disadvantages. One is accessibility. Mechanics often have no choice but to lie under the machine and manipulate tools to release the brake. Furthermore, the operator is not given any feedback as to whether the brake has been fully released or not. And a machine stopped on an incline requires the brake to be released without the machine rolling down the slope at the same time — while a person is lying underneath the machine.

A procedure with an equally potential danger is the manual re-engagement of the parking brake, whether the vehicle is on a flat bed trailer, at the shop, or on the side of the road. Because the operator receives no indication (neither on the operator panel nor in any central location) as to the operating status of the brakes, all parking brakes must be inspected visually. Should the operator forget to unscrew the spindle or reset the eccentric tappet, the machine can still be operated even though the machine’s parking or emergency brake cannot be applied.