Selecting rolling bearings for industrial gearboxes

By working together early in the design process, gearbox manufacturers and bearing suppliers can reduce development times and improve the overall safety, quality and reliability of the end product, says Dr Steve Lacey, Engineering Manager at Schaeffler (UK) Ltd.

Developing precision rolling bearings for industrial gearboxes has never been more challenging. Like most engineered components and systems today, gearboxes are becoming smaller - especially in automotive applications - but also more powerful. The supporting bearings must therefore offer optimum performance.

Refer to diagram on the right: Schaeffler's rolling bearing analysis and calculation programme BEARINX® is a software tool that enables engineers to simulate bearing designs for complex gearbox systems, quickly and accurately.

The bearings must be compact, reliable, durable and offer high load ratings. In addition, gearbox designers will also look for other factors including low noise levels, reduced friction, as well as how safe and straightforward the mounting of the bearings will be.

In the UK, Schaeffler supplies bearings to a number of different gearbox manufacturers. Typical applications for these gearboxes include wind turbines, tidal energy systems, planetary gearboxes for power generation, construction vehicles, racing car transmissions, as well as gearboxes for gas, water and vacuum pump applications.

Typically, the gearbox manufacturer provides a design specification and/or drawings to the bearing supplier. Most (around 85%) of Schaeffler gearbox bearings are standard catalogue products. The remainder are custom designed bearings to suit specific applications. Here, the customer often requires careful guidance and technical support from the bearing supplier.

At this point, important decisions are made and design factors that determine the suitability of a bearing for a specific application are discussed. These include the space available for the bearing; the loading the bearing has to withstand; the speed at which the bearing needs to perform; the environment in which the bearing will operate; lubrication requirements; and the required life expectancy of the bearing.

By working closely with an engineer from a specialist bearing supplier such as Schaeffler, gearbox manufacturers can select the most appropriate bearing for their application. For some applications, it may simply be a question of mounting the appropriate bearing in the correct manner in order to ensure optimum performance. However, if the bearings are only considered much later in the design process, the customer may have already signed off important parts of the machine design, without giving much thought to the bearing requirements. As a result, there may not be sufficient space to mount the ideal bearing or lubrication method. This means the bearing engineer has to select a compromised bearing solution that may not meet all of the customers' requirements.

Collaborative design
The ideal scenario is therefore to invite the bearing supplier into your design process early, in order to avoid potential costly design errors, over- or under-engineering the product and to eliminate safety or reliability issues that will need rectifying later on. Product development times will be reduced considerably if bearing design issues are resolved early.

Furthermore, a standard bearing (which is always more preferable to the customer) is more likely to be recommended for the gearbox if the customer starts the design with a clean sheet of paper. If the design of the gearbox is already set in stone when the bearing supplier is invited to specify bearings, the chance of a special, custom-designed bearing solution being offered is higher.

The collaborative design process starts with a detailed brief from the customer, which is passed to Schaeffler's in-house engineering team. These engineers use the latest design software - including kinematics, dynamic, FEA analysis software and its own, unique BEARINX© calculation software - to simulate the function of the proposed bearing or system. This results in a bearing design that achieves the required performance and operating life.

Model complete gearbox systems
Schaeffler's rolling bearing analysis and calculation programme BEARINX® is a software tool that enables engineers to simulate bearing designs for complex gearbox systems, quickly and accurately. The software enables the rapid modelling of complete geared systems, taking into account important design factors such as elasticity and deformation. Calculations and design simulation help to improve the understanding of the overall system, so that critical points can be identified and eliminated at an early stage in the design process.

BEARINX® models the complete power transmission system in a gearbox, taking into account the type of gears used, loads, speeds and existing collective loads. Gear shafts that deform elastically under the given loads based on materials used, are also taken into account.

For example, the elastic deformation of the gear unit housing can have a significant influence on bearing life. Combining BEARINX® with FEA calculations for gearbox housings makes it possible to include the rigidity of the housing in any design analysis. As well as examining static conditions of the gear unit, dynamic effects on the bearings and shaft can also be simulated.

Reports from BEARINX® can be emailed to the customer. 3D models of the geared drive can be included here. Different operating scenarios such as changing the load or speed conditions can be inputted to BEARINX® and the results analysed graphically if required.

In a recent customer application, Schaeffler UK used BEARINX® to analyse 53 separate bearings in an agricultural gearbox. 73 different gear selections and configurations were made on the four-wheel drive system. This simulation model took four weeks to set up and then seven hours (overnight) to run the software calculations.

Selecting the "best bearing for the job" and "fit-for-purpose" is always the objective at Schaeffler. Understanding exactly what the customer (and end user) requires is therefore critical.
Schaeffler works with most of the world's leading gearbox manufacturers. Flender AG, for example, for many years has used Schaeffler's large tapered roller and spherical roller bearings on the input and output sides of its gearboxes, which offer very high load ratings. The intermediate shafts also use cylindrical roller bearings from Schaeffler.

For planetary gearboxes, Schaeffler supplies companies such as Bosch-Rexroth with compact, cylindrical roller bearings, single or double row, with or without special coatings. Tapered roller bearing pairs and double row angular contact ball bearings have also proved themselves as robust main bearing supports in planetary gearboxes.

Renk AG uses ball bearings and cylindrical roller bearings from Schaeffler to support the shafts on large extruder gearboxes, with power ratings as high as 27,000kW. Schaeffler's tandem bearings are also used for supporting the very high axial loads on the worm shafts.

High precision gearboxes, manual gearboxes and some planetary gear units require needle roller bearings. Ball bearings can be supplied for geared motors, tapered cylindrical gear units, worm gearboxes and cylindrical gear units (input shaft, coupling). Schaeffler also provides axial bearings for extruder, marine and mill gearboxes.

For demanding industrial gearbox applications, Schaeffler offers its premium quality X-life brand of bearings. For example, the X-life full complement cylindrical roller bearings consist of machined outer and inner rings and rib-guided cylindrical rollers. Because these are designed with the maximum possible number of rolling elements, these bearings offer very high radial load carrying capacity and high rigidity, which makes them ideal for compact gearbox designs. When used as semi-locating bearings, they also support axial loads.

Premium quality roller bearings
Due to their large contact angle, Schaeffler's X-life single row tapered roller bearings T7FC can support high radial loads, as well as axial loads in one direction. Two bearings in an 'O' or 'X' arrangement can support radial forces and moments, as well as axial forces from both directions. With X-life optimised surfaces, these bearings offer a 20% higher dynamic load rating, resulting in a 70% increase in the basic life rating of the bearings.

For further information, e-mail: info.uk@schaeffler.com or view website: www.schaeffler.co.uk    Refer to next page

Schneider Electric urges machine builders
to utilise guidance when using EN 954-1

With the European Commission's news to allow EN 954-1 to continue for a further two years, many machine builders will carry on using this standard to support compliance to the Machinery Directive, despite the introduction of EN ISO 13849-1 and EN 62061. However, Schneider Electric is warning if the standard is not used correctly, they could be wasting thousands of pounds on 'over specification', as well as compromising on their machines' safety.

Peter Still, industry standards manager at Schneider Electric, explains: "Despite EN 954-1 having been published many years ago, there is still widespread misuse which stems largely from the so-called 'risk graph' in an informative annex to the standard. What is often not understood is this graph illustrates just one small aspect of the standard and is not a full summary of the requirements. Consequently, those focussing on this element alone are not complying correctly and therefore not achieving the safest machines possible.

"By not reading the standard in its entirety, machine builders are not addressing the risks correctly and often assigning too high a category - spending more money than is necessary. For instance, a single category might have been wrongly assigned to a machine, when in fact if the standard had been read fully, the category would be selected for each safety-related part of the control system according to the degree of risk reduction provided by that part, thus optimising both the safety aspects and overall cost of the control system."

To help those operating in the industry to understand EN 954-1 and how to comply, Schneider Electric is urging them to read a European Committee for Standardisation (CEN) report, CR 954-100, which although not widely known, has been available for some years and gives useful guidance on how to correctly use the standard.

Peter continues: "While the 'risk graph' can be a useful tool, it is an illustration of just one small part of the process of using the standard correctly. With the introduction of EN ISO 13849-1 and EN 62061, now is a good time to reassess how a machine's safety is achieved. Those sticking to using EN 954-1 will find that if they do so correctly, the transition to the new standards further down the line, particularly EN ISO 13849-1, will be much easier as it uses some similar basic principles.

"Better still, there is no reason why machine builders shouldn't start using the new standards now, as they focus on the correct operation of safety functions, making it possible to achieve greater levels of safety throughout the machine's life. In any case, it will be necessary to stop using EN 954-1 before the withdrawal of its presumption of conformity in 2011."

The CR 954-100 report is available from the British Standards Institution as PD CR 954-100:1999 or for further information on this report view the Schneider Electric website: www.schneider-electric.co.uk and follow the links to Machinery Safety Legislation, click Support, and then follow the BSI safety standards link.