20.08.2009 | Author / Editor: C.B. Ray, India / Marcel Dröttboom
Some of the characteristics of hydraulic motors make them especially suitable for being used as bulk material handling equipment. However, identifying the appropriate type for a certain task is not as trivial as it may seem.
Hydrostatic drives have gone a long way in bulk material handling. High power density made it universally acceptable in mobile machines. It is also popular in wide range of stationary equipment where speed and torque control are of great importance. Low moment of inertia, high starting torque and smoothness add to the other features of hydrostatic drives.
The construction and mechanism characterise the features of hydraulic motors. Some motors can deliver very high torque at very low speed, but have limitations at high speed. Others, which operate at very high speed, deliver low torques and show ripples at low speed. Some motors are very compact and have an extremely high power density but are not very efficient and have problems in handling high starting torque.
Variation of displacement or variation of torque at constant pressure, which is a unique feature of hydraulic motors, is not available with all types due to the limitations in mechanism. Configuration also plays an important role in some applications. In some cases motors become more efficient when combined with gearboxes, especially planetary gearboxes. The torque capacities of same size (i.e. displacement) of different types vary widely due to their different pressure rating. Naturally, all these factors need to be considered for the selection of a motor.
Hydraulic motors of the different types have their own characteristics and areas of application. An overview with some examples is given in Table 1. Unlike hydrodynamic turbines, all positive displacement motors contain torque/speed ripples which become more distinct with decreasing speeds. The cam opposed axial piston and cam ring type radial piston motors almost eliminate the ripples even at speeds below 1 rpm and are mostly used for gearless direct drive.
One of the greatest advantages of the hydrostatic drive is its lower mass compared to electrical or electro-mechanical drives. But different mechanisms involved in the hydraulic motors lead to differences in mass in each category. The chart in Fig. 1 compares the mass of different categories. A common parameter mass per unit of displacement is compared, considering motors with solid shafts. It is observed that orbit motors have the lowest and gear motors have the highest mass per displacement.
Volumetric and overall efficiency depend on the construction and working principle, but for each category volumetric efficiency is high at higher speed range and falls with decreasing speed. The internal leakage through the clearances in a motor is predominantly dependant on the working pressure and remains almost constant at a particular pressure, irrespective of flow or speed. Consequently, the volumetric efficiency is less at lower speed where the volumetric flow is smaller. The variation in volumetric efficiency with respect to speed is illustrated in Fig. 2. Piston motors are exposed to a smaller area of clearances compared to other types and, therefore, have higher volumetric efficiency with smaller rate of reduction with respect to speed. Hence, they are rated at high operating pressure and suitable for high starting torque applications.
Generally, these types of equipment do not have much space constraint, especially not in the radial direction. For belt conveyers speed is in the moderate range, but belt feeders and apron feeders run at very slow speed. In all these cases starting torque requirement will be high and for belt and apron feeders the drive has to manage jamming or close to jamming situations imposed by hopper load. For conveyors, braking is very important and the motor should have a dynamic as well as a static braking arrangement. Overhung load need not be considered for hollow shaft motors. Choices in those cases are direct radial piston - cam ring, eccentric shaft and axial piston type motors combined with a gearbox, preferably planetary. Choice obviously will also depend on the availability of sizes in the particular category. Brakes can be built-in or separately mounted on the drive shaft. Whenever a gearbox is used, there is a possibility of reducing the size of brake.
Here the bucket wheel drive will require high starting torque and the ability to withstand frequent overloading and jamming situations. For mounting the drive, there is limitation of space in axial direction, but sufficient space is available in radial direction. So radial piston - cam ring design is the ideal choice. However, a combination of a bevel helical or bevel planetary gearbox with an axial piston motor may also be considered.
For slew drives, limitation of space is in radial direction. Combination of planetary gearbox with axial piston or radial piston-eccentric shaft type is popular. Here, the installation of a braking device is extremely important.
Using hydraulic motors is not yet much popular in long travel drive systems. Probably this is due to high cost. If judiciously examined dual displacement cam ring with or without a single stage planetary gearbox could be good solution. Installed power rating and power consumption can be reduced. A common power pack with single prime mover can also make the drive system compact and simpler.
The drive requires very high positive torque at starting and negative during discharge while tippling. There are high torque surges and consequent pressure peaks. Motors capable of managing such conditions are radial piston - cam ring type or axial piston combined with gearbox. Still now the convention is to drive through a pinion and sector gear by two or three hydraulic motors. Direct drive at the trunion-shaft, eliminating pinions and sector gear is not yet economic.
The drive system normally requires very high starting torque for positioning of the full rake of wagons. Some times there are a few braked wagons and track alignment is not proper. To deal with such a situation sufficient margin in torque is necessary in the drive unit. The motors are radial piston with or without gearbox and axial piston with gearbox.
Rotations of plough arms in coal heap require high starting torque. The obvious choice is radial piston - cam ring / eccentric shaft with or without gearbox and axial piston with gearbox. Whenever the arms are directly mounted on the output shaft, care is taken so that shaft and bearing of drive unit withstand radial and axial loads. Configuration of motor/drive unit is also important so that it does not interfere with the hopper walls.
Mobile machines like these extensively use variable/dual displacement motors for travel drive. In both track drive and wheel drive, automatic transmission or control of speed depending on torque is possible by use of step less variable displacement axial piston motor.
In bulk material handling applications most of the motors are of high torque and slow speed, except belt conveyors, where speed is high compared to other applications. It is seen that piston motors including radial and axial are the only suitable ones which deliver high torque and have the capability to handle overload conditions. Orbital, gear and vane motors are mostly used in equipment accessories. For example, orbital motors are common in cable reeling drums to offer a constant tension to the cable. Gear and vane motors are used in fan/compressor drives for low torque and high speed requirement.
In case of high torque and low speed applications where high starting as well as overall efficiency are required, following may be the solution:
Radial piston (cam ring) with or without reducer
Radial piston (eccentric shaft) with or without reducer
Axial piston (cam opposed) with or without reducer
Axial piston (bent axis) with reducer
For the particular application of a apron feeder different alternatives are compared in Table 2. The data compared may vary to some extent from one manufacturer to other but they show a trend. Three columns at the end of the Table indicating efficiency, mass and moment of inertia are important to note and useful for selecting the type of motor.
Axial piston (cam opposed) motors are used for slow speed applications and mostly limited to direct drive in mobile machines. Bent axis/swash plate type axial piston motors are basically high speed motors, but in combination with 3- or 4-stage gear reducers, these are also used in low speed high torque applications. Due to the wide variety of displacement control, they are very popular in mobile applications. The gear reducers connected to such motors are subjected to very high input speed, which may reduce the life of gears in the 3rd or 4th stage.
For very slow speed applications, e.g., below 1 rpm, radial piston motors coupled with a gear reducer may be used. If a gear reducer is not preferred, the cam ring type is the only solution. But since all motors loose efficiency at their lower speed range, it is appropriate to run the motors at their optimum operating speed range.
Table 2 shows that at lowest speed, efficiency falls to approximately 70 percent. The cam ring motors which are designed and rated for a speed range of, say 50 rpm to 70 rpm, if used for lower speed range will operate smoothly without any functional inadequacy. But at fractional rpm efficiency falls drastically. Hence, there is a need for motors especially rated for lower operating speed range, say for example 0.05 - 5 or 0.1 – 10 rpm. Target for efficiency may be 85 percent to 90 percent comparable to gearbox motor combination.
Earlier it was observed that for high torque and heavy duty applications, higher size cam ring motors used as direct drives were heavier than the combination of single stage planetary gearbox and lower size motor. The moment of inertia was also higher in case of the former. But today this does not hold good in general. Motor manufacturers have reduced the mass and moment of inertia of their products drastically through internal research and development. They are now comparable to any gearbox motor combination which is seen in Table 2.
In material handling high starting torque and peak torque requirement (due to overload or jamming) for short duration are mostly taken care by keeping a margin on normal operating pressure in fixed displacement motors. When the margin is not sufficient, higher displacement is chosen resulting in higher size pump.
Variable displacement or dual displacement motors can manage such situations efficiently. Mobile machines are taking the advantage of this feature for a long time. This reduces the capacity of pump, size of valves and other components and also installed power. Axial piston is the only type available in this category which can offer continuous step-less variation in displacement. Radial displacement motors can offer stepped variation or dual displacement.
In apron feeders, belt feeders, and plough feeders higher displacement mode will take care of high torque requirement at slow speed and lower displacement mode will be effective for normal running at lower torque.
In side arm charger, while pulling the full rake of wagons, where torque requirement is high, initially displacement may be at the highest range and as the number of pulling wagons reduces, displacement may be reduced progressively. While pushing empty wagons or during idle movement motor displacement may be kept at its lowest range.
In most of the material handling applications where high starting torque and slow operating speed are the primary requirement, cam ring type radial piston motors without any reducer are the ultimate solution. Now the challenge to this type of motor manufacturers is to introduce series of motors which have reasonably high efficiency at the lowest speed range, comparable to or better than that of motor gearbox combinations.
On the other hand efforts are needed to use variable or dual displacement motors as far as possible to reduce the size of power pack, prime mover and power consumption.n
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