Optimum fusion of rear axle and steering assembly.
With a lift capacity of up to two tonnes, Cesab's electrically powered forklift trucks are well acquainted with heavy loads. These vehicles are widely used by small and medium sized companies, where they are required to perform a wide range of internal handling operations day after day. Because of the difficulty of working in confined spaces, the manoeuvrability of forklift trucks in this weight class is a key competitive factor. When they designed and built their latest generation of forklifts, Cesab relied on the expertise of gearbox specialist Bonfiglioli to make two sizes of steering axle.
To lift and move loads of up to two tonnes in confined spaces in absolute safety are decisive requirements in relation to the design of the exterior of a forklift truck. Additionally it is important to solve the problem of how to obtain positive and responsive steering. In comparison with conventional axle construction, Cesab was looking for enhanced manoeuvrability, a reduced steering radius, and more efficient manufacturing solutions in terms of costs. The forklift maker opted for a three-wheel chassis with two front drive wheels and one rear steer wheel. Equally important was the decision to mount twin wheels in order to optimise load distribution for the rear steering axle design solution - the two wheels are so close together that they effectively function as a single wheel.
The ideal partner for the development and production of the steering axle was soon identified. Departing from the customary solution of working with vehicle chassis specialists, Cesab decided to adopt an alternative approach. The drives specialist was asked to come up with a solution also for the steering system. This is a logical conclusion, because it means that the forklift manufacturer purchases all components of the chassis and the drive from a single partner. Bonfiglioli rose to the challenge, assigning the project to its internal department specialised in drive technology for mobile equipment. Thanks to 10 years of experience in the field of low speed vehicles and in hydraulic drives technology and working together with their colleagues from Cesab, the experts from Bonfiglioli Trasmital soon came up with a solution that satisfied all requirements.
The compact construction of the rear steering axle was achieved as follows: a stable support with multiple screw fixings serves to robustly connect the axle to the forklift truck structure. This support has the dual function of forming a housing for the hydraulically operated steering mechanism and supporting the bearings for the vertical connection to the axle proper. Two taper roller bearings support the vertical load transmission while simultaneously guaranteeing unrestricted rotation capacity. The vertical connection is mechanically configured to allow rotation through a range of more than 90°. The wheel axle is installed at the lower extremity and is also held in two taper roller bearings and closed by the two wheel hubs.
The forklift truck steers by rotation of the vertical connection with the necessary steering power generated by a hydraulic jack. Linear movement of the piston, which slides in both directions in the cylinder, is converted to angular steering movement by means of teeth cut directly in the piston, meshed with a pinion that is an integral part of the vertical connection with the axle proper. The solution adopted makes it possible to dispense with the use of any additional moving parts or costly articulated construction solutions. And this means savings in both space and cash.
The available rotation angle of more than 90°, in combination with the front drive wheels, constitutes a major benefit for the small lift truck. If the truck reaches full lock of the rear steer wheel, the front wheels start to rotate in opposite directions. This means that the centre of rotation is located between the two front wheels rather than on just one of them, depending on the steering angle. The result is maximum manoeuvrability in confined areas. This function is made possible by the vehicle's electronic controller.
There is a potentiometer mounted on the top of the steering kingpin, connected to the shaft by means of a slip ring. Electrical signals generated on the potentiometer in proportion to the steering angle are measured by the lift truck's electronic controller, which adjust the speed of both the front wheel motors in accordance with the results. The controller therefore performs the role of an electronic differential.
In the words of Bonfiglioli's Germany branch manager, Thomas Becker: "A steering axle for a forklift truck would appear at first sight to be an unusual product for a drives specialist, but this is not the case at all. We take the utmost pains to get to know exactly what demands our customers' machines must fulfil. On this basis we then apply our know-how to the advantage of our customers, reaching also beyond the specific field of drives."
In the sector of mobile equipment drive technology, current thinking is increasingly geared towards tailored solutions for specific machines. Projects are carried forward in close liaison with the customer, and manufacturing takes place in a regime of optimised costs and just-in-time supplies.
For more information contact Bonfiglioli Power Transmission, +27 (0)11 608 2030, [email protected], www.bonfiglioli.co.za
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