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.
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.
Another simple solution to disengage the brake of a disabled vehicle is to use a ball valve and a hand pump. The ball valve serves as a connection between brake valve and brake cylinder. The hand pump can then be used to pressurize the brake’s hydraulic line, thereby releasing the brake. This method saves time because, depending on the hydraulic circuitry, it disengages all brake cylinders simultaneously. Perhaps more importantly, it does not require a mechanic to crawl under the machine. This makes it possible to safely release the parking brake even when the machine is parked on an incline.
The position of the ball valve at a central position of the machine indicates the operating status of the parking brake. Some machine manufacturers fix the control lever in place with a screw in order to prevent the ball valve from being actuated inadvertently. This prevents unintentional disengagement of the parking brake. However, a mechanic could still forget to reset the ball valve once repairs are complete or when the towing procedure is finished. As with the mechanical release, this would leave the vehicle’s parking brake disabled.
Indicating the switching position of the ball valve to the operator requires monitoring pressure in the brake line or position of the valve actuating lever. Because it is rarely necessary to interfere with the parking brake circuit, this type of monitoring is generally not implemented. Therefore, no convenient method is provided to determine whether or not the parking brakes have been disengaged.
The most effective solution would be to provide quick and easy disabling of the brakes, prevent unintentional disabling, and provide feedback so an operator knows if the parking brakes are operational or have been disabled. However, to be practical for machine builders, it cannot add substantially to the cost beyond that of the standard hydraulic solution.
Automatic pressure-dependent resetting of the changeover valve has been developed by Argo-Hytos, Bowling Green, Ohio. The solution is both simple and cost-effective. It does not interfere with the machine‘s control system and can be retrofitted into existing machines or integrated into new designs.
The Argo-Hytos solution works in a fashion similar to the standard hydraulic valve already described. It incorporates a manually actuated 2-port, 2-position (2/2-way) directional control valve fitted with a hydraulic resetting system. Pushing a button causes the 2/2-way directional valve to block flow between the brakes’ cylinders and the main 3/2-way brake valve. A hand pump is then used to pressurize the line to the brake, which has now been isolated from the rest of the hydraulic circuit.
When the towing procedure is finished, the parking brake can be reengaged by pulling out the control button. If the operator forgets to reset the 2/2-way directional valve after repairing the machine, the valve will be reset automatically by the brake supply pressure. This resetting takes place regardless of whether or not the actual brake valve has been actuated or de-energized because the control signal in the pressure line is tapped upstream of the 3/2-way directional valve. This means the machine cannot be operated if the parking brake has been disabled unintentionally. The operator can be confident that the parking brake is operational simply by noticing that the control button is extended. This solution greatly improves the safety, efficiency, and reliability operation of mobile equipment.
An integrated solution
The valves are housed in a manifold block, which can also incorporate the hand pump. Three models are available for different applications. Model 1 consists of a pressure-switched 2/2-way directional control valve and a pressure relief valve. It is intended for machines already equipped with a hand pump. Model 2 is identical to Model 1 but it incorporates a hand pump into the manifold. Model 3 features a hand pump, a 2/2-way directional and relief valve, plus a solenoid actuated 3/2-way directional valve, which serves as the main parking brake control valve. This full-featured assembly is a cost-effective alternative to specifying all the components separately, takes up little additional space, and is easy to install.
This information was provided by Argo-Hytos. For more information, call (419) 353-6070 or visit www.argo-hytos.com.