Perhaps due to their simplicity, worm gear designs are typically selected for relatively uncomplicated applications, such as conveyors, where precision requirements have traditionally been low, and their compact form makes installation straightforward. Many gearboxes for these applications are built with cost as a primary driver; they offer acceptable performance and durability for their purpose, but limited efficiency and high, unpredictable levels of backlash preclude them from use in more demanding, high-precision applications.
Automation requirements are changing, however. Today's flexible conveying and handling systems are expected to offer much higher levels of performance. Conveyors must operate in both directions, at high, controllable speeds and with great precision. To deliver the right kind of control capability, designers have been abandoning worm gearboxes for in-line gearheads with higher efficiency, less backlash and more resistance to wear. Along the way, they have also had to make expensive compromises in terms of space efficiency in order to fit other gearbox designs into their equipment.
Now designers can exploit an alternative option. High-precision right-angle servo gearboxes offer extremely good durability and very low backlash for accurate control. Historically, such desirable properties have come at too high a cost for less demanding conveyor control applications, but new designs mean that it is possible to select the right combination of precision, durability and cost for almost any application.
For the most challenging roles – such as robot control, machine tools and very high-precision handling - ZF's WT range uses a hypoid gearbox designed and manufactured to high precision. In typical applications they can offer a service life of 30,000 hours and backlash as low as 2arc/min.
High-precision servo applications
ZF's latest innovation delivers accuracy and durability that is nearly as high as the WT range, at considerably lower cost. The SWG right-angled gearbox is said to be one of the first wormwheel gearboxes designed specifically for high-precision servo applications.
At the core of the new SWG technology is a computer-optimised profile that delivers 90 per cent contact between worm and wheel – in contrast, conventional designs only offer around 40 per cent contact. The broader contact area minimises wear, ensuring that the gearbox has a long life and that backlash levels remain constant and predictable. A further benefit of the design is exceptionally low noise levels in use, less than 55dB.
The wheel itself is manufactured from a specially developed, highly wear resistant bronze alloy. This material is demanding to machine, but is far more resistant to wear and pitting than cheaper, softer materials. Throughout the SWG gearbox, similar design details serve to increase capacity, durability and precision: large-diameter bearings offer increased load bearing capacity; and the wormwheel is securely mounted to the shaft with a press fit and multiple pins – an arrangement that is far more durable than the conventional keyed fit.
Most appealing for designers juggling demanding cost and accuracy constraints, the SWG gearboxes are available in three levels of accuracy, allowing exactly the right balance to be struck. The high-precision version has backlash of less than 1arc/min, the precision box offers 3arc/min and the standard unit limits backlash to 10arc/min.
Options for new-build and retrofit applications
The SWG units can operate at input speeds of up to 6000rpm and output torques of up to 6900Nm. They are available in ratios from 5.2:1 to 90:1, in ten different sizes and with four output options for maximum flexibility. Sizes 30-800 are available with dual output shafts, making them particularly suitable for conveyors and handling systems.
Space-optimised aluminium-magnesium alloy cases make the gearboxes highly suitable for weight-critical automated assembly and packaging systems. The many configurations available in the range mean that off-the-shelf gearboxes are available for an extremely wide range of applications, offering designers the flexibility of a custom gearbox at the cost of a standard unit.
In use, SWG gearboxes offer service lives of around 25,000 hours. This high durability, combined with the units' efficiency and quiet performance is making them increasing popular in retrofit applications on existing conveyor installations. Many designers are now taking advantage of these benefits even in relatively simple conveyor designs too, where they offer lower cost of ownership than cruder, 'disposable' alternatives.
