Drive options are a topic of discussion guaranteed to provoke disagreement and confusion amongst design engineers and crushing plant users alike. Traditionally, the crusher would be V-belt driven directly from the diesel prime mover, decoupled by a mechanical clutch. Due to the inertia of the flywheels, the operator would have to fiddle the clutch lever to coax the machine to start without stalling the engine.
This worked well in the past but the market is now crowded with other solutions claiming to be the best way and it can be confusing for plant users to know what will work best.
Omitting torque converters and hybrid diesel-electric drive solutions, which are not in widespread use due to their cost, the industry gravitates around dry plate clutch and hydraulic drives.
MECHANICAL DRIVES
In terms of absolute efficiency, the dry plate clutch has the lowest power loss of all methods, if fuel burn is the only consideration. On the downside, the trade-off is pure efficiency versus functionality.
To offer a soft-start clutch engagement, computer-controlled pilot systems are the norm for modulating or ?fiddling? the engagement (as the operator used to do). As every start raises the clutch plate temperature, its thermal capacity limits the permissible number of starts per hour. Torsional vibration from the engine can cause ?micro-slips? which raise running temperature and can lead to premature failure.
Depending on the arrangement of clutch main shaft bearings, limited side-load capacity can leave the clutch and engine sensitive to over-tensioning of the drive belts and as the drive belts have to be tensioned by moving the entire engine pack, correct tensioning can be difficult. Also, the direct V-belt drive limits the available positions for the engine installation.
As mobile plant with varying feed size and quality can be prone to blockage, the crushing chamber of a direct drive machine must be cleared manually if a blockage was to occur. If an attempt to start a blocked crusher is made by clutch engagement, serious damage may occur. While some machines can open the setting to discharge the blocking material, it is unlikely that a re-start is possible without manual intervention.
The dry plate clutch occupies the primary power take-off on the prime mover. A tracked mobile plant will require an additional high torque power take-off to drive track pumps. This leads to some ingenious designs to accommodate the requirement for power take-off by way of external drive splitter boxes, crankshaft nose power take-offs and forward belt drive and cardan shafts. Original equipment accessory drives which are fitted behind the engine bellhousing raise the pumps high, making it difficult to properly flood feed the pump inlet port and leading to cavitation in cold weather.
It should be remembered that the crusher drive is a part of the whole system. A wasteful auxiliary drive system can lose any efficiency advantage gained by the drive method.
HYDRAULIC DRIVES
Hydraulic drives have been used for crusher drives by several major manufacturers with a good history of success. The major benefits of a hydraulic system relate to greater flexibility in both control and operation.
While the OEM cost of a hydraulic transmission system can exceed a dry plate clutch, the main system pump will supply hydraulic power to the crusher and tracks for mobility. The main pump occupies the primary power take-off of the prime mover and no secondary high torque power take-off is required, resulting in a compact installation.
As the crusher drive is indirect, the drive train does not constrain the engine and pump assembly location. The engine may be mounted in the best position and orientation for effective operation and maintenance.
Modern transmission pumps can be controlled by an on-board computerised system. In starting the crushing plant, the flywheels may be accelerated up to speed in seconds without imposing excessive load on the engine and doing it almost silently. The control system monitors the engine working parameters in real time and if the power draw becomes excessive, the pump delivery can be reduced to enable maximum productivity to continue uninterrupted.
What is the difference between hydraulic and hydrostatic drives? A conventional open loop hydraulic system draws oil from a reservoir, through a high pressure pump, and then drives the motor. The return flow from the motor passes back to the reservoir. This technique allows the motor to be reversed using a directional control valve and is protected from overload by a pressure relief valve. The arrangement is shown in Figure 1.
{{image3-a:L-w:350}}{{image4-a:L-w:350}}In the hydrostatic system, high pressure oil from the pump passes to the motor then returns to the pump inlet in a continuous closed loop. This means that if the load tries to run faster than the pump, the engine is used as a brake to maintain control. By these means, the crusher can not only be driven by the system but can be slowed and stopped. This arrangement is shown in Figure 2.
A further advantage of the hydrostatic system is that the pumps can be permanently controlled by the machine?s on-board computer.
Typically, when a mobile crushing plant is shut down, the flywheels continue to rotate for several minutes. Having smoothly accelerated the flywheels to working speed, the hydrostatic drive system can now bring them smoothly to a stop in a few seconds. An additional benefit to this degree of control is that ?emergency stop? can mean just that – it brings the entire plant safely to a stop!
Having stopped the machine in an emergency, the jaw will be left full of rock. A clutch drive machine would have to be manually cleared; however, a hydraulic machine can reverse jaw rotation and start up, the jaws still full, without danger of damage. Clearing a blockage is one of the most dangerous procedures when operating a crushing plant. With hydraulic blockage clearance, it becomes a routine which is performed remotely and with reduced danger. The reverse crushing action can also be used for crushing delicate materials such as asphalt which are susceptible to packing, as well as blockage clearance.
Precise control also manifests itself in tracking. A heavy plant can now steer by varying the speed of each track independently as well as slewing on the spot. It is not necessary to constantly shock the plant by slewing just to take a curve in the roadway; it can be steered by running one track faster than the other.
Given the degree of electronic control, to assist the operator a direct data link may be fitted to enable remote access from the manufacturer. Not only can performance data be recovered for breakdown situations but modifications may be made to the programming to adapt the machine?s performance to the operator?s needs.
But what is the downside? Hydraulic machines are accused of inefficiency but a well designed system can offer advantages which offset reduced efficiency. It is true that a simple gear pump and motor combination when hot and heavily loaded can get down to 50 to 60 per cent efficiency, but a modern axial piston transmission pump/motor is quite different. The individual component efficiency can exceed 90 per cent, providing the system is well designed and thermal management is effective, as the system is most efficient over a small range of oil temperature.
Older open-loop transmissions were filled with over 1000 litres of oil, frequently dumped and replenished. This is no longer the case as state of the art, closed loop hydrostatic drives have a lower reservoir capacity and condition-based maintenance based on oil sampling can significantly reduce maintenance costs.
IMPROVED FUNCTIONALITY, DURABILITY
While dry plate clutch technology continues to develop with intelligent engagement systems and improved bearing capacity, the attractions of an efficient transmission system are obvious, providing the entire plant has been optimised for maximum efficiency.
Hydraulic systems, however, offer greatly improved functionality and durability with very low routine mechanical maintenance and will continue to appeal, especially the new generation hydrostatic machines which enjoy enhanced control and safety features unavailable on mechanically driven machines.
Source: CMB International Ltd
This article first appeared in the September 2011 issue of Quarry Management (UK) and appears in Quarry with kind permission.