In significant industrial segments across the globe, there continues to be major investment in large-scale equipment that incorporates heavy-duty material movement and handling systems. In mining applications such as bucket wheel excavators and conveyors, to metal shredders in recycling operations, as well as winch systems and drilling equipment in offshore applications, powerful low-speed drives that can deliver high torque and reliably function in rugged demanding environments are in high demand.
There are several kinds of drive platforms that are suitable for this. However, a growing number of operations are finding that the hydraulic direct drive (HDD) provides the performance these systems require. HDD systems offer significant performance advantages for applications where a heavy mass needs to be moved under variable speeds using a system that can handle “shock loads” (sudden increases in the weight and mass of loads being moved), combined with the ability to deliver energy-efficient and reliable performance — often operating 24 hours a day, seven days a week.
Hydraulic Direct Drive Technology
The most common use of HDD systems is for industrial applications moving heavy masses on a continuous basis with low speed and high torque, and especially high starting torque for operations with frequent stops and starts. The materials being moved can be done at low speeds, typically in a range from zero to 200 rotations per minute (RPMs).
These systems also perform well in applications where “shock loading” occurs, where large heavy loads are dropped onto moving conveyors, feeders, crushers or turning drums, suddenly varying the load size by several tons during normal operations. The drive must be able to respond to the shock load without undue wear and tear on drive components.
HDDs provide that performance because of their unique design — most specifically because they are “direct drives” that deliver the full energy of their operation to the shaft they are driving. An HDD is a closed system with a low-speed hydraulic motor at its heart. Able to sustain high torque even at minimum speed, the hydraulic motor is mounted directly on the drive shaft — there is no need for a gear reducer, belts, chains, or sprockets.
Power is supplied to the hydraulic motor by a separate drive unit, which can be positioned almost anywhere in relation to the installation. The drive unit contains at least one standard AC induction motor, which runs at a fixed speed and drives a variable-displacement axial piston pump. It is the variable flow of oil from the pump that determines the speed and direction of the drive.
The complete HDD system also includes the intelligent pump controller, hydraulic fluid supply, and connecting hoses and wiring. The power unit is connected to the hydraulic motor on the shaft via cabling and hosing; this lets system designers position the pump, electric motor, and controllers in an enclosure away from the operational axis. This allows for greater design flexibility and protects these components from harsh operating conditions.
Comparing Drive Technologies for Heavy Equipment
HDD systems are being more widely used in heavy equipment applications; nevertheless, there are other drive systems in use to provide the same function. More traditional industrial drives are generally available in two platforms: a medium-or high-speed drive motor, which can be either hydromechanical or electromechanical, combined with a gear reducer to provide the low-speed, high-torque operation.
Hydromechanical drives (HMDs) have basic torque and speed that are similar to those of an HDD, but the drive is connected to the drive shaft of the system through a gear reducer. This configuration creates mechanical losses that reduce the output torque. Exactly how much torque is lost depends on the type of gear reducer used, the number of gear stages it has, and the factor by which it has been overdimensioned.
A variable-speed AC drive (ACD) combines a high-speed AC induction motor with a gear reducer to achieve a low running speed. In some cases, a fluid coupling must be installed between the motor and gear reducer. There are several different methods of controlling an ACD. These methods usually allow a controllable speed to range from 0 to 100 Hz in heavy-duty applications. When running at the rated frequency of 50/60 Hz, the drive can operate continuously at 100 percent of the motor’s rated torque; however, at lower speed the continuous torque available is reduced.
Advantages of HDD Platforms
There are several inherent advantages that HDD systems offer compared to other systems. With a geared drive solution, the electric motor, gear reducer, and coupling are mechanically connected to the driven shaft, often requiring considerable space around the machine. A direct drive is much more compact. Equally important is that close to 97 percent of the full power of the HDD system is applied directly to turn the driven shaft. A geared drive solution will also generate high additional torque in case of sudden speed changes caused by heavy shock loads. Failure to compensate for these can lead to considerable costs from breakdowns and resulting downtime. A direct drive eliminates this risk.
This can help preserve equipment that may undergo punishing shock loads during normal operations. For example, many mining conveyors use ACD drives to move tons of material through their processes. When these types of drives start up, the starting inertia is immediately applied to the belt, which over time can strain the belt and lead to breakdowns.
In contrast, an HDD will provide the full torque at zero speed immediately upon start-up. This lets mining operators slowly advance the speed from one RPM through very tight increments so that the belt is not strained yet the material is transported efficiently.
The simple, rugged design of hydraulic motors also enables them to start, stop, and reverse repeatedly without taxing the interior cams and other motor elements. This enables an operator to design the motion sequences to fit the load or work requirements of the machine the HDD is driving, stopping and starting as many times as is needed. With an ACD or HMD that uses a gear reducer, to perform the same number of starts and stops, the gearbox would need to be overdimensioned to handle those conditions without quickly wearing out the drive — and overdimensioning a gearbox can be an expensive solution.
HDD systems also offer superior speed and directional control compared to gearbox-couple drive solutions. Since the variable flow of oil from the pump determines the speed and direction of the drive, speed and directional control are not compromised by the limitations of the electric motor. And due to the hydraulic motor’s low moment of inertia, the response is almost instantaneous.
Along with the shock load resistance inherent in the technology, HDD technology also provides overload protection to safeguard machines and ensure maximum uptime. HDDs provide an adjustable pressure-limiting function, which prevents maximum torque from being exceeded in overload conditions. Maximum torque is prevented from being exceeded by destroking the pump as necessary during lengthy acceleration cycles and in overload conditions.
This offers greater flexibility, enabling the operator to respond to different production or operational requirements. For example, a recycler may be shredding plastic on a shredder one day and choose to reduce the HDD’s maximum pressure for that process. That same machine could be used a few days later to shred metal; in that instance, knowing that more torque will be needed, it is a simple control change to increase the maximum pressure allowable without taxing the drive.
Choosing Complete HDD Solutions
There are a few ways heavy equipment end users can implement HDD systems in their operations. There are some hydraulic motor suppliers who can provide the motor that mounts on the drive shaft; it would then be up to the end user or a hydraulics system integrator to obtain the variable-speed drive, hydraulic pump, controller, and other drive components and program the system to start-up.
Some hydraulics suppliers offer complete solutions in turnkey packages. Some suppliers also provide expert recommendations, installation support and effective repair services. Partnering with an expert supplier helps ensure that the right set of drives is engineered and installed on each piece of heavy-duty equipment to deliver the full advantages of HDD technology for the full system life cycle.
This article was written by Brian Howell, Large Hydraulic Drives Sales and Operations Manager, Bosch Rexroth (Bethlehem, PA). For more information, visit here .