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One of the current trends in materials handling is the use of tow tractors and tugger trains, which in many plants are replacing the forklift trucks employed previously. The driving force behind this development is the fact that material flows can be better structured using the new vehicles. The tow tractors use defined stopping stations to pick up products or drop them off, allowing a greater degree of automation or semi-automation.
As a medium-sized specialist in high-performance hydraulic drives, Gerhard W. Ruppel Hydraulik of Bad Münder, in Germany has developed and built a vehicle of this type, which harnesses the power density of hydraulics to provide semi-automatic loading and unloading of large loads. The vehicle has been successfully trialled in a car battery manufacturer’s factory.
The battery manufacturer has to transport pallets loaded with finished products and weighing up to 1.5 tonnes. The firm cannot use fixed conveyor technology because of the structural system supporting the building. In any case, conveyors would not allow them the flexibility required by their production processes.
The concept of the vehicles arose when the manufacturing engineers decided they wanted to move to a higher level of automation, based on the fact that the batteries are produced in large batches. To help achieve this ambition, Ruppel Hydraulik developed the concept of a platform truck for three pallets having a maximum load weight of 4.5 tonnes (Figure 1). The platform is split into three sections, each of which can take one pallet. This is a standard design. What is special about this vehicle is the transverse conveyor belts which the pallets stand on and which allow semi-automatic loading and unloading.
In use the operator positions the tow tractor with the trailers parallel to a handover station or a conveyor belt. He can then convey a pallet onto the belt using the truck’s transverse conveyor system, without the need for fork-lift trucks or other vehicles.
Hydraulic power
The transverse conveyors of the tugger trailer which ensure properly sequenced material flows are hydraulically powered. To achieve this, Ruppel Hydraulik designed a power pack which can be fitted as a compact underfloor unit, one advantage being that within a small space it can deliver the high power required to start up the conveyor belts with their heavy loads (Figure 2). To operate the drive the operator uses a control panel which is fitted to the front end of the platform truck.
The valve block of the unit is based on the use of standard CETOP valves. This keeps costs to a minimum and makes integration simpler. As is typically the case for hydraulics, the intelligence resides in the switching interfaces and linking of the various elements involved, including the safety engineering, as the conveyor belts are not the only components of the platform truck which are hydraulically powered.
Each conveyor belt is protected on both side of the platform truck by a chock rail. This is a metal element which can be extended upwards from the vehicle frame thereby ensuring that the pallet cannot slide off, particularly when the truck has to turn a tight corner. This safety system also ensures that, once picked up, the pallet remains in the central position, i.e. within the confines of the conveyor belt. If the chock rail cannot be fully extended because it encounters an obstacle (generally this will be an incorrectly positioned pallet), to supplement the visual check of the operator, there is a sensor fitted which can also detect this and trip a warning signal. Sensors also detect the limit positions of the chock rails.
This results in the following motion sequence. When the operator wants to drop off a pallet, he inserts the safety key in the key switch on the control panel (Figure 3) and uses the selector switch to activate the desired section of the truck. Then he can start the motion. First the chock rails along the loading side are retracted, then the conveyor belt moves in the desired direction and deposits the pallet. Once the motion has been completed, the chock rails are extended upwards again.
These particular motions, which are intended for safety purposes, are all performed hydraulically. It goes without saying that all relevant requirements of the Machinery Directive and the trade and professional associations are met.
Flexible and robust
The decision to opt for a hydraulic rather than an electric drive has the added benefit that a central drive unit can execute a variety of functions and the conveyor belt drives can be easily linked to the chock rail drives. High power, deliverable at short notice, is thereby provided from within a compact installation area, and the entire drive system is extremely robust.
In addition, it is really easy to adjust the drive parameters via the hydraulics, with all control being executed through a PLC (programmable logic controller). If the characteristics of the material flow or pallet weights should happen to change, the desired acceleration and deceleration values of the belt drives as well as the positioning of the belt can be quickly changed with the minimum of fuss.
Ruppel Hydraulik says that while the platform trucks were developed for a user in the field of battery manufacturing, the concept, which extends the function of a tow tractor and trailer tugger trains to include semi-automated unloading, is of course ideally suited to other industries.
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