Integrating absolute angular measurement into the bearing
07 June 2015
Schaeffler has developed an angular measuring system suitable for direct-driven, highly dynamic, high precision rotary axes in machine tools. The system incorporates an inductive (absolute) angular measuring system integrated in a rotary table bearing.
The principle of integrating the SRM measuring system into a rotary table bearing was successfully implemented in Schaeffler’s YRTM range of bearings, but has now been developed further. The SRM measuring system combines high measuring speed and measurement accuracy with a reduced design envelope and increased robustness.
The system is insensitive to contamination (such as lubricants and coolants), offers a rotary axis with an open centre (freeing up space in the centre of the machine) and reduces alignment errors. Moreover, the free space in the centre of the axis can be used for mounting other machine ancillary components. With fewer mechanical components, the system is easier to replace and maintain.
Schaeffler’s new YRTSMA bearings can be fitted to the rotary axes as an alternative to optical angular measuring systems. YRTSMA bearings are available with bore diameters from 150mm up to 1,030mm.
High measurement accuracy
By fitting the YRTSMA bearing to the rotary axis, information about the absolute angle is available immediately after switching on the machine. With an incremental measuring system, absolute position cannot be found until a reference run is carried out after switch-on.
Knowledge of the absolute position is critical if there is a risk of collision between the tool and the workpiece (such as may be the case when projecting edges collide) during the reference run. This system is therefore typically used in milling head applications.
The measuring scale of the YRTSMA series is positioned directly on the inner ring. Thanks to the excellent radial run-out of the measuring scale, which is manufactured in high precision bearing steel, a second measuring head is not required. By using only one measuring head, the overall design envelope is significantly reduced, without adversely affecting system performance.
With their reliance on the inductive measuring principle, YRTSMA bearings are less sensitive to the rolling bearing grease and contaminants that are normally present in machine tool rotary table applications. The electronic evaluation system is integrated into the measuring head so that the system can be connected directly to the machine tool control system.
The measuring head is effectively sealed off from the adjacent construction and is designed to be screw-mounted to the bearing, which means it is positioned at the correct distance from the measuring scale at all times, without requiring any adjustments. The measuring head is easily accessible and can be replaced without additional set-up if required.
Plain bearings compendium
Also new from Schaeffler is a 428-page catalogue describing its range of plain bearings. The catalogue contains information on ELGES radial, angular contact and axial spherical plain bearings, as well as rod ends, metal/polymer composite plain bearings (in the form of plain bushes), thrust washers/strips, and ELGOTEX plain bushes. The catalogue is a fully revised version that replaces three previous catalogues: 238, 706 and ELGOTEX plain bearings.
The ‘Technical Principles’ section provides guidance on the selection of plain bearing for different applications. A number of important design factors are considered, including load carrying capacity, rating life, friction, increases in temperature, internal and operating clearance, sealing, mounting and dismounting, and ISO tolerances.
The catalogue also contains updated chapters on ‘Special Materials’ and ‘Special Designs’. The Special materials section includes examples of non-standard materials that can be specified for spherical plain bearings such as glass fibre-reinforced plastic sliding layers.
Other examples include E60, a new plain bearing material from Schaeffler for maintenance-free film bearings. This metal/polymer composite comprises a bronze rib mesh that is filled with a dry lubricant made from PTFE with embedded chemically non-reactive additives. The mesh functions as both a carrier and a sliding layer.
Another example is E421, a two-layered metal/polymer composite comprising a steel supporting body with a plastic composite layer made from PTFE. The extremely thin-walled material facilitates very small design envelopes. Potential products include bushes, flanged bushes, thrust washers and strips. The material comprises a steel backing and a sliding layer. As standard, the steel backing is protected against corrosion by a zinc coating.
The ‘Special Designs’ chapter includes examples of custom-engineered plain bearings such as plain bushes with slots, pivoting cradle bearings, ball cups, bush and pin systems, linear plain bearing units, flanged spherical bearings, prop shaft centring bearings, central joint bearings, and spherical plain bearings with bellows.
Throughout the catalogue are examples of X-life plain bearings. These are premium quality bearings that utilise higher performance materials, lower coefficients of friction and reduced running-in wear when compared to conventional, standard plain bearings. For a free copy of ‘Plain Bearings’ (publication HG 1), email: firstname.lastname@example.org
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