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Sliding bridge bearings
Traction lubricant solves clutch problem Tolartois SA manufacture perforated steel goods for the electrical and building industries at their plant at B~thune in northern France. Clutch problems on a multi-head punch pressing machine resulted in below target output and high maintenance costs.
"TOP"
The Wegner punch press uses Formsprag H P I 600 series indexing roller clutches to convert reciprocating motion into intermittent rotation of a feed roll system, advancing the steel rapidly and precisely. Using the specified mineral oil, Tolartois were unable to achieve perforation accuracy of -+0.01 mm at the required indexing/punching rate of 400 cycles/min.
I n d e x i n g sprag clutches c o n v e r t reciprocating m o t i o n into unidirectional rotation. The d u t c h m u s t run f r e e l y in o n e direction, while "locking-up' in the other
Why it w o r k s
Lubricant changed One fairly expensive solution to the problem would have been to change the clutches to higher rated units. Tolartois decided, however, to try a different lubricant. In a preliminary trial Santotrac 50*, a synthetic hydrocarbon based traction oil, was used to fill two worn clutches which were giving neither the required precision nor indexing rate after running for 10-12 million cycles. Clutch action was restored, and both performed over 45 million cycles before being removed for overhaul. During the trial the lubricant was changed weekly as recommended by the clutch manufacturer. Following this trail, all the sprag clutches were filled with Santotrac 50 and the oil drain period was extended to one month. The required perforating precision and indexing rate were achieved; targetted minimum clutch life of 30-40 million cycles has been exceeded; and maintenance and down time costs have been reduced. * Santotrac is a registered trade mark o f the Monsanto Company. The Santotrac range includes four oils and two greases. Monsanto Ltd, 10-18 Victoria Street, London, S W I H ONQ, UK or Monsanto Industrial Chemicals Co, 800 N Lindbergh Boulevard, St. Louis M e 63166, USA
204
Improved clutch performance using Santotrac 50 can be attributed to the physical properties of the lubricant. Current theory I indicates that, under the high pressures and shear rates commonly encountered in elastohydrodynamic contacts, the viscosity of the lubricant increases until there is a transition to a 'glassy solid'. The
transition is, of course, affected by other variables including temperature, which tends to inhibit 'solid' formation. Early work on the formation of glassy solids related mainly to longer molecules. Recent data, however, supports application of the idea to mineral oils 2 and it has been shown that with increasing pressure the behaviour of the lubricant approximates to that of an elastic solid. Contact between rolling elements is prevented by the solid 'pad', reducing wear. The pad also reduces relative slip. It has been suggested that the advantage of Santotrac over mineral oils is that the solid produced has a higher shear strength. Also, the manufacturers claim that Santotrac has "an u n u s u a l l y high transitory viscosity-pressure increase" and state that "contact pressures of 700 000 psi have not succeeded in destroying the integrity of the film between rolling metal-to-metal surfaces".
References Traction Theory Moot. Imperial College, London. December 1976 2.
Conference on Concentrated Contact Lubrication. N A S A Lewis Research Centre March 1977
Sliding bridge bearings Articulation of the 68 span Cromarty Bridge, which will carry the A9 trunk road, will be accommodated by 450 floating piston bearings.* Each bearing is designed to carry a total load up 1037 kN.
The feasibility of this bearing design, which has been developed during the past decade from original work carried out in Germany, is largely due to the unique combination of characteristics of PTFE.
into which a close-fit natural rubber disc is sealed. The piston, a steel disc, is positioned in the base casting directly on the rubber disc: it has a 2.5mm deep circular recess machined in its upper surface into which a circular disc of 4 mm thick virgin PTFE is placed. The steel upper member o f the bearing has a 3 mm thick stainless steel slider plate bonded and mechanically attached.. This stainless steel plate, finished to 0 . 1 - 0.15 microns, slides against the PTFE.
A base casting of spheroidal graphite iron forms a shallow cylindrical pot
Performance
Supplied by BTR Solarbridge, BTR Silvertown Ltd, Horninglow Road, Burton-on-Trent, Staffordshire, UK
Since the rubber disc has no force free surfaces and is contained in a rigid member, given sufficient pressure due
Bearing design
*
TRIBOLOGYinternational August 1977
to compressive load it acts hydrostatically. This permits the upper members of the bearing to float and rotate (_+0.01 radians) about any horizontal axis, thus accommodating the flexure under load of the "simplysupported" bridge deck spans. The effect of the hydrostatic action of the rubber disc on the PTFE is to maintain a reasonably uniform pressure distribution. This is an essential feature of the bearing in that it minimizes the moments transmitted to the supporting piers. The bearing will permit up to +_12mm of movement in any horizontal direction. The dead load to be carried is 313 kN with additional live load up to 724 kN per bearing. The resulting mean pressures on the PTFE being 9.7 N / m m 2 and 3 2 . 0 N / m m 2 respectively. A considerable amount of research work has been carried out in the UK 1,a and in Germany to establish the coefficient of friction which may be expected from bridge bearings with these materials. Results indicate that, without lubrication, the initial coefficient of friction at normal temperature and uniform contact pressure of the order 30 N / m m 2 will be less than 2.5°70. As the bearing 'runsin', the coefficient of friction will drop to less than 1.5070. Use of suitable silicon based lubricants has been shown to considerably reduce the friction. The Cromarty Bridge bearings will be lubricated to provide an initial reduction of the friction level. Test carried out in BTR Silvertown's Laboratories show that, in this form at normal temperature, a coefficient of friction of approximately 1% can be expected at contact pressure of 20 N / m m 2. A point of particular interest is that the coefficient of friction of virgin PTFE/stainless steel surfaces reduces with increasing contact pressure whether dry or lubricated.
Pi~ plate sting
Floating piston bridge bearing provision of initial lubricants probably reduces, if not prevents, PTFE transfer in the early working life of bearings. A particularly relevant property of PTFE in this application is its chemical inertness. When used with high quality stainless steel, PTFE provides a fatigue resistant and maintenance free sliding mechanism.
References 1.
Long ]. E. Performance of PTFE in
bridge bearings. Civil Engineering and Public Works Review Iiol 64 No. 754, 1969, 459-462 2.
Holland D. A. Bridge Bearings, Civil Engineering October 1976, 44-48
3.
Design with 'Fluon' PTFE. ICI Publication pp 6-7
Flow and fatigue Cold flow of virgin PTFE is restrained to a negligible level by the recess into which it is seated and wear of the contact surface in the life of the bearing is not expected to exceed 0.5 mm. Tests to measure wear 1,3 indicate that the running-in process and some of the initial wear may be due to transfer of PTFE to the stainless steel surface thus tending to a P T F E / P T F E sliding couple. The
Pilot installation o f an automatic constant level oiling system at Shell Chemicals UK Ltd's Stanlow plant has worked successfully f o r over 8 months. Oil feeds by gravity f r o m a central reservoir to the glass reservoir o f each oiler. A float operated shut-off valve maintains a constant level in each oiler and hence in each pump, gear-box, etc. G. E. Adams and Co. Ltd, Broad Lane, Coventry, CV5 7AZ, UK
TRIBOLOGYinternationalAugust 1977 205
"TOP"
The Wegner punch press uses Formsprag H P I 600 series indexing roller clutches to convert reciprocating motion into intermittent rotation of a feed roll system, advancing the steel rapidly and precisely. Using the specified mineral oil, Tolartois were unable to achieve perforation accuracy of -+0.01 mm at the required indexing/punching rate of 400 cycles/min.
I n d e x i n g sprag clutches c o n v e r t reciprocating m o t i o n into unidirectional rotation. The d u t c h m u s t run f r e e l y in o n e direction, while "locking-up' in the other
Why it w o r k s
Lubricant changed One fairly expensive solution to the problem would have been to change the clutches to higher rated units. Tolartois decided, however, to try a different lubricant. In a preliminary trial Santotrac 50*, a synthetic hydrocarbon based traction oil, was used to fill two worn clutches which were giving neither the required precision nor indexing rate after running for 10-12 million cycles. Clutch action was restored, and both performed over 45 million cycles before being removed for overhaul. During the trial the lubricant was changed weekly as recommended by the clutch manufacturer. Following this trail, all the sprag clutches were filled with Santotrac 50 and the oil drain period was extended to one month. The required perforating precision and indexing rate were achieved; targetted minimum clutch life of 30-40 million cycles has been exceeded; and maintenance and down time costs have been reduced. * Santotrac is a registered trade mark o f the Monsanto Company. The Santotrac range includes four oils and two greases. Monsanto Ltd, 10-18 Victoria Street, London, S W I H ONQ, UK or Monsanto Industrial Chemicals Co, 800 N Lindbergh Boulevard, St. Louis M e 63166, USA
204
Improved clutch performance using Santotrac 50 can be attributed to the physical properties of the lubricant. Current theory I indicates that, under the high pressures and shear rates commonly encountered in elastohydrodynamic contacts, the viscosity of the lubricant increases until there is a transition to a 'glassy solid'. The
transition is, of course, affected by other variables including temperature, which tends to inhibit 'solid' formation. Early work on the formation of glassy solids related mainly to longer molecules. Recent data, however, supports application of the idea to mineral oils 2 and it has been shown that with increasing pressure the behaviour of the lubricant approximates to that of an elastic solid. Contact between rolling elements is prevented by the solid 'pad', reducing wear. The pad also reduces relative slip. It has been suggested that the advantage of Santotrac over mineral oils is that the solid produced has a higher shear strength. Also, the manufacturers claim that Santotrac has "an u n u s u a l l y high transitory viscosity-pressure increase" and state that "contact pressures of 700 000 psi have not succeeded in destroying the integrity of the film between rolling metal-to-metal surfaces".
References Traction Theory Moot. Imperial College, London. December 1976 2.
Conference on Concentrated Contact Lubrication. N A S A Lewis Research Centre March 1977
Sliding bridge bearings Articulation of the 68 span Cromarty Bridge, which will carry the A9 trunk road, will be accommodated by 450 floating piston bearings.* Each bearing is designed to carry a total load up 1037 kN.
The feasibility of this bearing design, which has been developed during the past decade from original work carried out in Germany, is largely due to the unique combination of characteristics of PTFE.
into which a close-fit natural rubber disc is sealed. The piston, a steel disc, is positioned in the base casting directly on the rubber disc: it has a 2.5mm deep circular recess machined in its upper surface into which a circular disc of 4 mm thick virgin PTFE is placed. The steel upper member o f the bearing has a 3 mm thick stainless steel slider plate bonded and mechanically attached.. This stainless steel plate, finished to 0 . 1 - 0.15 microns, slides against the PTFE.
A base casting of spheroidal graphite iron forms a shallow cylindrical pot
Performance
Supplied by BTR Solarbridge, BTR Silvertown Ltd, Horninglow Road, Burton-on-Trent, Staffordshire, UK
Since the rubber disc has no force free surfaces and is contained in a rigid member, given sufficient pressure due
Bearing design
*
TRIBOLOGYinternational August 1977
to compressive load it acts hydrostatically. This permits the upper members of the bearing to float and rotate (_+0.01 radians) about any horizontal axis, thus accommodating the flexure under load of the "simplysupported" bridge deck spans. The effect of the hydrostatic action of the rubber disc on the PTFE is to maintain a reasonably uniform pressure distribution. This is an essential feature of the bearing in that it minimizes the moments transmitted to the supporting piers. The bearing will permit up to +_12mm of movement in any horizontal direction. The dead load to be carried is 313 kN with additional live load up to 724 kN per bearing. The resulting mean pressures on the PTFE being 9.7 N / m m 2 and 3 2 . 0 N / m m 2 respectively. A considerable amount of research work has been carried out in the UK 1,a and in Germany to establish the coefficient of friction which may be expected from bridge bearings with these materials. Results indicate that, without lubrication, the initial coefficient of friction at normal temperature and uniform contact pressure of the order 30 N / m m 2 will be less than 2.5°70. As the bearing 'runsin', the coefficient of friction will drop to less than 1.5070. Use of suitable silicon based lubricants has been shown to considerably reduce the friction. The Cromarty Bridge bearings will be lubricated to provide an initial reduction of the friction level. Test carried out in BTR Silvertown's Laboratories show that, in this form at normal temperature, a coefficient of friction of approximately 1% can be expected at contact pressure of 20 N / m m 2. A point of particular interest is that the coefficient of friction of virgin PTFE/stainless steel surfaces reduces with increasing contact pressure whether dry or lubricated.
Pi~ plate sting
Floating piston bridge bearing provision of initial lubricants probably reduces, if not prevents, PTFE transfer in the early working life of bearings. A particularly relevant property of PTFE in this application is its chemical inertness. When used with high quality stainless steel, PTFE provides a fatigue resistant and maintenance free sliding mechanism.
References 1.
Long ]. E. Performance of PTFE in
bridge bearings. Civil Engineering and Public Works Review Iiol 64 No. 754, 1969, 459-462 2.
Holland D. A. Bridge Bearings, Civil Engineering October 1976, 44-48
3.
Design with 'Fluon' PTFE. ICI Publication pp 6-7
Flow and fatigue Cold flow of virgin PTFE is restrained to a negligible level by the recess into which it is seated and wear of the contact surface in the life of the bearing is not expected to exceed 0.5 mm. Tests to measure wear 1,3 indicate that the running-in process and some of the initial wear may be due to transfer of PTFE to the stainless steel surface thus tending to a P T F E / P T F E sliding couple. The
Pilot installation o f an automatic constant level oiling system at Shell Chemicals UK Ltd's Stanlow plant has worked successfully f o r over 8 months. Oil feeds by gravity f r o m a central reservoir to the glass reservoir o f each oiler. A float operated shut-off valve maintains a constant level in each oiler and hence in each pump, gear-box, etc. G. E. Adams and Co. Ltd, Broad Lane, Coventry, CV5 7AZ, UK
TRIBOLOGYinternationalAugust 1977 205