pressure comb

The disadvantage of helical gears, which are commonly used today, is that in addition to a radial bearing force, there is also a component in the axial direction, which must be supported by the gear bodies, shafts, bearings and housings. High loss forces occur in the axial bearings in particular, which reduce the efficiency of such gearboxes.

Pressure meshing, on the other hand, are rings attached directly to the gearing with which the gearwheel and pinion are supported axially against each other. The power flow is thus closed for a much shorter time than with conventional bearings. This means that gears, shafts and, above all, bearings can be designed to be lighter, which minimizes power losses. In addition, external axial forces can be transferred from the high-speed shaft to the low-speed shaft, where they can be dissipated with fewer losses via a thrust bearing.

Even if there are no general calculation specifications for pressure combs, they are state of the art in turbine gearbox construction. They are also used in marine gearboxes and to a limited extent in vehicle gearboxes. At the IMW, work has been and is being carried out to create general calculation rules in order to enable a wider use of pressure comb technology.

The pressure comb was first patented by the Swiss company BBC in 1922. At that time, it was still attached as a separate machine element next to the gear pairing. A disk attached to the pinion shaft rotates in a circumferential groove that runs around the wheel shaft at approximately the same height as the gearing. The friction contacts are located to the right and left of the groove.

In the course of further development, the design that is still common today, in which the pressure combs enclose the gearing, became established. This minimizes problems caused by rotor dynamics, especially in high-speed gearboxes.

In the eighties of the last century, a commercial vehicle transmission was equipped with pressure meshing for the first time. This made it possible to switch from spur gearing to helical gearing, which resulted in an average noise reduction of 10 dB.

The Institute of Mechanical Engineering at Clausthal University of Technology has been researching pressure combs for years. To date, extensive work has been carried out on the deformation behavior and running of the rings. At present, the focus is shifting to the investigation of the frictional contact, in particular the start-up behavior is to be investigated.

Technology

Pressure combs are usually designed as two rings on the left and right of a gear pairing. They form light cones to create a converging lubrication gap. Due to the line contact, high pressures occur, so that the elasto-hydrodynamic lubricating film theory is used for the calculation. The oil is supplied via the oil injected into the gearing.

Due to the very small lubrication gap, the pressure comb is sensitive to deformation of the contact partners. This can be adjusted either by a rigid design or by a rounded surface of one of the friction partners. The line contact then becomes a point contact.

Furthermore, the axial load capacity of the pressure combs is limited. Depending on the installation method, however, there are ways of preventing the hub from gaping open and thus increasing the load capacity.

In addition, as with all hydrodynamic bearings, there is the problem of passing through the mixed friction area at low speeds without causing damage. If the performance is not sufficient for this, the occurrence of damage can be reduced by suitable surface treatment.

Current research work

Two main areas are currently being investigated at the Institute of Mechanical Engineering:

Deformation analysis

On the one hand, the deformation behavior of thick circular ring plates subjected to eccentric axial force is being investigated. The findings from this project lead to an analytical design guideline for thin, wide hubs that are designed as interference fits. In addition to pressure combs, wide pinions also fall into the category of eccentrically loaded thin hubs.

The tests are carried out on a static deformation test rig, which is examined using 3D coordinate measuring technology. In addition, numerical calculations are carried out, which are compared with the experimental results. Both investigations are used to develop an analytical calculation formula based on Kirchhoff's plate theory.

Wear tests

A two-disc test rig is used to investigate the influence of different types of pressure comb design on wear during start-up. At average circumferential speeds of up to 5 m/s, axial forces of up to 120 kN can be applied. In continuous operation, differently designed pressure comb test specimens can be examined until the surface fails. The results are used to derive design guidelines for damage-free continuous operation.

This investigation is also accompanied by numerical simulation in order to be able to apply the results to a larger parameter field. In addition, a basis for further investigations of the operating behavior is to be laid.

Literature

Dietz, P. ; Mupende, I.:Pressure comb - an old machine element with new application potential. In: Konstruktion 58 (2006), No. 4, pp. 69-75

Thoden, D.: Elasto-hydrodynamic lubrication of pressure combs. In: Dietz, Peter (ed.): Institute Communication 2006. Clausthal-Zellerfeld, 2006, p. 23-26

Thoden, D.: On the deformation of thick circular ring plates under eccentric axial load. In: Lohrengel, Armin; Müller, Norbert (Eds.): Institute Communication 2009. Clausthal-Zellerfeld, 2009, pp. 5-12

Thoden, D. ; Lohrengel, A. ; Dietz, P.: Aktuelle Entwicklungen in der Druckkammauslegung. In: Schlecht, Berthold (Ed.): DMK 2009 - Dresdner Maschinenelemente Kolloquium : November 24 and 25, 2009. Dresden : TUDpress, 2009

Brown, Boveri & Cie AG: Spur gear with one-sided helical gearing; Patent 401652 (1922)

Murch, L. ; Wilson, W.: A Thermal Elastohydrodynamic Inlet Zone Analysis.In: Transactions of ASME, Journal of Lubrication Technology 97 (1975)

Dowson, D. ; Higginson, G.: Elasto-hydrodynamic Lubrication. SI ed. Oxford : Pergamon Press; Pergamon Pr., 1977

Kehl, G. ; Mertin, F.: Lubricating film load carrying capacity in pressure comb gears. In: Tribology and Lubrication Technology 48 (2001) (2001), No. 1, pp. 12-15

Langer, H.: Hydrodynamic axial force transmission in shafts of high-speed gearboxes. In: Konstruktion im Maschinen-, Apparate- und Gerätebau 34 (1982) Heft 12, Seite 473-478

Contact us

For further information, please contact us at info@imw.tu-clausthal.de.