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PRELIMINARY STUDY OF FRICTION BETWEEN ICE AND SLED RUNNERS
K. Itagaki, G. Lemieux, N. Huber
To cite this version:
K. Itagaki, G. Lemieux, N. Huber. PRELIMINARY STUDY OF FRICTION BETWEEN ICE AND SLED RUNNERS. Journal de Physique Colloques, 1987, 48 (C1), pp.C1-297-C1-301.
�10.1051/jphyscol:1987142�. �jpa-00226288�
PRELIMINARY STUDY OF FRICTION BETWEEN ICE AND SLED RUNNERS
K. ITAGAKI, G. E. LEMIEUX and N. P. HUBER*
U.S. Army C o l d R e g i o n s R e s e a r c h and E n g i n e e r i n g L a b o r a t o r y , 7 2 , Lyme R o a d , H a n o v e r , NH 0 3 7 5 5 - 1 2 9 0 , U.S.A.
a art mouth C o l l e g e , H a n o v e r , NH 0 3 7 5 5 , U.S.A.
Re'sumB. Le coefficient d e friction d e l a g l a c e a e'td mesure'par l a ddc6le'ration d'un t r a v e a u (bob-sled) glissant sur une couche d e glace obtenue e n chambre froide. En cours d e glissade, l'augmentation d e la t e m p 6 r a t u r e e n t r a i n e une baisse du coefficient d e friction du patin "tendre,"
cependant pour un patin dur I'augmentation 16gbre du coefficient d e friction semble indiquer que l a the'orie des lubrifiants liquides e s t applicable s e u l e n e n t a u matdriau tendre.
Abstract. The e f f e c t s of runner m a t e r i a l and s u r f a c e conditions on t h e friction between runners and i c e w e r e studied by measuring t h e velocity of a free-sliding sled. Smooth runners showed lower friction a t around - l ° C t h a n around -lO°C a s expected, but t h e friction of rough runners showed l i t t l e t e m p e r a t u r e dependence.
1. Introduction
Anybody who has stepped on a patch of ice recognizes t h a t ice can be very slippery. Bruises and broken bones a r e commonplace among careless pedestrians who walk on ice. The poor trac- tion of t i r e s on icy roads has caused many fata1,automobile accidents. On t h e brighter side, t h e slipperiness of ice makes i t possible t o enjoy winter sports such a s skiing, skating and sled- ding and t o transport heavy cargoes over ice and snow by sled.
The most popular explanation f o r t h e lo\v friction of ice is lubrication by a thin layer of water between t h e slider and t h e ice melted f r o m t h e ice e i t h e r by t h e h e a t of friction (1) o r by high pressures (2).
T u s i n a (3) measured t h e friction between a s t e e l ball and ice a t speeds low enough t o preclude fluid lubrication. He explained t h a t t h e coefficient of friction p between solid materials and ice, expressed a s
p = shear adhesive strength/compressive yield strength,
is low because t h e shear adhesive strength of ice around t h e melting point is much smaller than i t s compressive yield strength.
If lubrication by a liquid film is required t o reduce drag, thinning of t h e w a t e r layer due t o leak- a g e must b e t a k e n into account. Furushima (4) used a highly simplified roughness profile t o cal- c u l a t e t h e liquid film support and t h e r a t e of squeezing under speed skates, and found t h a t liquid lubrication was e f f e c t i v e at roughnesses lower than 0.2 pm o r perhaps e v e n 0.05 pm.
The studies mentioned above a r e based on t h e o r e t i c a l models and on testing accomplished under idealized conditions. Our intention was t o identify t h e basic problems involved in ice resistance incurred under t h e dynamic conditions of real sledding f o r immepiate application by t h e U.S.
Olympic Bobsled Team. Such resistance would include not only frictional resistance but also energy dissipation processes such a s groove formation, t h e crushing of ice protrusions, founda- tion vibration, etc.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987142
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2. Experimental
A small sled weighing 60 kg was propelled across a n i c e s h e e t grown in a coldroom. We tried t o simulate real bobsled run conditions a s closely a s possible in regard t o runner surface, s t r e s s under t h e runner, and ice conditions. The velocity was limited t o less than 1/20 of a n a c t u a l bobsled's speed due t o t h e space limitation.
The model sled (Fig. 1) had a pair of runners mounted in parallel grooves on runner holders a t - tached t o a 30- x 30-cm-square aluminum plate. A s e t of lead blocks provided c o n t a c t s t r e s s levels under t h e runners comparable t o those on a real bobsled. The runners were made of $-in.
round rod material; both front and r e a r ends were rounded. Different s e t s of runners made of rhree different materials (see Table 1) and surface t r e a t m e n t s could be mounted by loosening t h r e e bolts t h a t fastened each runner t o i t s holder. The runner materials had different hard- nesses, and i t was difficult t o produce identical s u r f a c e conditions f o r e a c h run. Abrasive paper and a l e a t h e r s t r o p were used t o smooth t h e runner surfaces. However, t h e depth of t h e grooves c u t with t h e abrasive paper differed f r o m one m a t e r i a l t o another, and t h e l e a t h e r s t r o p was not equally e f f e c t i v e f o r a l l m a t e r i a l surfaces. Also, abrasive substances accumulated on t h e ice surface roughened t h e smooth runners and smoothed t h e rough runners. Bulk runner temper- a t u r e was measured by thermistors a n d thermocouples inserted in 11-mm-deep wells.
Table 1. Properties of runner materials.
1010 low carbon s t e e l 304 stainless s t e e l MP35N
Runner no. 1,2,7,8,14,17
Thermal conductivity ( A ) 68 W/mK (CRC) Rockwell A hardness (H) 51 (as measured) Corrosion resistance Visibly poor
Notes Soft and easy t o
machine.
3,4,9,10,16 17 W/mK (CRC) 58 (as measured) Good
Intermediate val- ues of hardness and t h e r m a l con- ductivity.
5,6,11,12,13,15,18 I 0 W/mK (Latrobe) 71 (as measured) Good
Tough t o machine but allowed fine
s u r f a c e finish;
hard enough t o retain finish.
A 20-cm-long a l u ~ l i n u m s h u t t e r mounted parallel t o t h e direction of n o t i o n was used t o meas- ure t h e sled's velocity. The duration of t h e interruption of a narrow beam of light by t h e shut- t e r was measured by a photo sensor and a n electronic counter. By t h e measurement of initial (vi) and final (vf) velocity between optical g a t e s 4.6 m (L) a p a r t , a n average coefficient of fric- tion p was calculated through energy considerations:
where g is t h e gravitational acceleration.
A 4i-in. (1 1.43-cm) d i a m e t e r Parker-Hannifin pneumatic cylinder was used t o a c c e l e r a t e t h e 60-kg sled t o a suitable speed (1.5 m/s) in a minimal distance. An operator-controlled solenoid valve between t h e accumulator and t h e cylinder advanced or r e t r a c t e d t h e r a m of t h e cylinder.
A pair of polyethylene guides a t t a c h e d t o t h e ice container directed t h e sled during t h e f i r s t 30 c m of i t s travel. Although t h e guides were adjusted carefully, t h e course t a k e n by t h e sled could not be predicted beyond t h e f i r s t 2 m of i t s run, and only a f e w runs t r a c e d t h e s a m e t r a c k a s t h e preceding run. Soft spots on t h e ice s h e e t would deflect t h e sled, sometimes causing i t t o hit t h e wall or e v e n turn around. D a t a from such runs were rejected.
The sled was stopped a t t h e o t h e r end of t h e ice t r a c k by a shock absorber made of a pair of smaller pneumatic cylinders. A f r a m e of angle irons around t h e e n t i r e box strengthened it and a c t e d a s fenders.
The ice was grown in a plywood box (1x5 m x 0.2 m deep) lined with a s h e e t of polyethylene.
zen layer was scraped evenly and a thermocouple was inserted. Five more layers of ice, each about a c e n t i m e t e r thick, were grown. As a final touch, a clean rag s a t u r a t e d with hot water was dragged across t h e ice surface, resulting in a +-mm layer of ice. This process was then re- peated. Asperities c r e a t e d during testing were removed both by buffing with a floor polish- e r and by scraping manually t w o days before e a c h series of runs.
Shutter for
Fig. 1. Side view of t h e model sled.
The hardness and roughness of t h e runners were measured t h r e e t i m e s during t h e course of t h e tests. Changes in roughness of t h e 1010 s t e e l were d e t e c t e d and t h e runners were repolished o r re-roughened t o reestablish t h e original surface conditions. We also made replicas of t h e runner and ice surfaces on several occasions f o r qualitative observation under t h e microscope.
A t t h e beginning of e a c h testing session, t h e selected runners were a t t a c h e d t o t h e sled. For t h e raised runner t e m p e r a t u r e t e s t s , t h e sled, with t h e runners, was placed on a heated alumi- num plate t o warm up. 1,feasurements of ice, a i r and runner t e m p e r a t u r e w e r e made before e a c h series of runs began. For e a c h run, t h e compressed a i r pressure, interval of light interruption by t h e s h u t t e r at t w o locations, and t h e runner t e m p e r a t u r e s (last 33 series only) w e r e recorded.
A t t h e end of a sequence of runs with t h e s a m e runner, t h e a i r , ice and runner t e m p e r a t u r e s were recorded again. The ice t e m p e r a t u r e was usually slightly lower than t h e coldroom temper- a t u r e , probably because of sublimation of t h e i c e in t h e dry atmosphere.
3, Results General
The model sled was run more than 700 t i m e s over t h e ice s h e e t using 18 different surface-fin- ished runners made from t h r e e materials. These cylindrical runners displayed l i t t l e directional stability. Any imperfection on t h e ice surface, such a s a groove l e f t by a previous run o r a nat- ural asperity (isolated small ~ r o t r u s i o n ) , could cause t h e sled t o s t a r t sliding sideways, turn around or e v e n spin uncontrollably. When t h e sled e n t e r e d t h e second g a t e at a n angle t h e apparent length of t h e optical s h u t t e r was shortened, which caused t h e calculated final speed t o b e f a s t e r than t h e initial speed. These results were rejected. Generally, t h e standard deviation of t h e one series of measurements remained within 10% of p.
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We a t t e m p t e d t o obtain a t least t e n repetitive readings f o r e a c h combination of runners, sur- f a c e t r e a t m e n t and runner temperature. In total, about 100 runs were rejected a s meaningless.
Effect of runner t e m p e r a t u r e
In a n a t t e m p t t o find t h e e f f e c t of runner t e m p e r a t u r e on t h e coefficient of friction, t h e run- ners were h e a t e d on a warm aluminum p l a t e before a series of runs. As t h e runners cooled back down t o ambient t e m p e r a t u r e during subsequent runs a change in friction could be observed.
0.008
1010
Rough Runner 0.006
-10 -8 -6 -4 -2 0
Smooth Runner
L
I I I I
-10 - 8 -6 -4 -2 0
Runner Temperature ( O C )
Fig. 2. E f f e c t of t e m p e r a t u r e on coefficient of friction f o r rough and smooth runners. All ice t e m p e r a t u r e d a t a were presented.
One s e r i e s of t e s t s was done a t relatively high ice temperatures, around -5OC. T h e e f f e c t of t e m p e r a t u r e was not c l e a r due t o a narrow t e m p e r a t u r e range and t o o much s c a t t e r in t h e data.
At ice t e m p e r a t u r e s around -lO°C, t h e results w e r e m o r e meaningful. The e f f e c t of runner tem- perature on p is shown in Figure 2. For smooth runners, t h e higher t h e runner t e m p e r a t u r e t h e lower t h e friction, whereas f o r rough runners t h e opposite trend was found.
S c a t t e r was considerable in some h e a t e d runner tests, but a regression analysis over t h e e n t i r e s e t of d a t a shows t h a t runner t e m p e r a t u r e a f f e c t s rough runners differently than smooth run- ners.
A t t h e low-temperature end, rough runners sometimes displayed lower friction t h a n smooth run- ners. This result is surprising since we roughened t h e runners by pressing abrasive paper against a rotating runner so t h a t grooves were c u t perpendicular t o t h e direction of motion, hardly ad- vantageous f o r reducing t h e friction.
Effect of runner surface
Aside f r o m t h e tests with smooth and rough runners described in t h e previous section, t e s t s w e r e performed with more highly polished runners (HP), a s well a s with a pair of runners which had deep longitudinal s c r a t c h e s (L) made in t h e m with coarse sandpaper, and with a pair of scratched and stropped runners (LS). The t w o l a t t e r runners were intended t o have more directional con- trol than t h e perfectly round c o n t a c t s u r f a c e would offer. No quantitative measurements of directional stability w e r e made, but controllability seemed t o improve somewhat, though fric- tion increased, in particular f o r t h e runner t h a t was scratched only. Polishing t h e longitudinally scratched runner surface with t h e s t r o p and jeweler's rouge reduced t h e friction considerably.
On warmer ice, particularly with t h e I.lP35N, friction was lower a f t e r this t r e a t m e n t than when i t was polished a s usual, and friction was nearly a s low a s f o r t h e highly polished, stropped run- ner. Comparison of these friction d a t a is difficult since roughness measurements w e r e difficult t o obtain and roughness changed during t h e tests.
m on t h e ice sheet. Surface replicas t a k e n a t t h e front end of a runner m a d e of 1010 mild car- bon s t e e l a f t e r about 50 m showed t h a t s o f t materials such a s this suffer substantial abrasion.
Therefore, s u r f a c e conditions may not have been t h e s a m e e v e n in one series of t e s t s , in partic- ular f o r t h e s o f t materials. The surface conditions eventually became consistent a f t e r several series of t e s t s , but t h e runners no longer had t h e i r original roughness.
Effect of runner material
According t o frictional melting theory, materials with lower t h e r m a l conductivity should retain more of t h e h e a t generated by friction, thus melting m o r e ice and producing a thicker lubricat- ing water layer t h a t results in lower friction (5). Contrary t o these expectations, tIP35N, which had t h e lowest t h e r m a l conductivity among t h e t h r e e materials we used, generally showed t h e highest friction when polished a s usual (S) and when roughened (R).
However, when t h e runners were highly polished (HI?), those with lower t h e r m a l conductivity glided b e t t e r , both at -lO°C and at -3OC. HP35N, with t h e lowest t h e r m a l conductivity, showed t h e least friction in t h e highly polished condition, followed by t h e slightly more conductive 304 stainless s t e e l and then by t h e highly conductive 1010 low carbon steel. The lowest friction coef- ficients obtained in t h e e n t i r e study c a m e f r o m t e s t s a t -3OC using highly polished MP35N run- ners.
During most t e s t s comparing runners with t h e regular degree of polishing, 304 stainless s t e e l showed t h e l e a s t friction, e x c e p t a t -3OC, where 1010 appeared most slippery. Among rough runners, 1010 performed best, while MP35N yielded t h e highest friction coefficient.
This ranking may have been due t o t h e f a c t t h a t asperities on t h e runner surface, which would increase t h e friction, a r e most easily abraded on a s o f t e r material such a s 1010 o r even 304, r a t h e r than a n extremely hard one such a s IiP35N. O n e would thus e x p e c t a gradual lowering of t h e friction in rough runner t e s t s with t h e s o f t 1010. However, such a trend is n o t c l e a r in our data.
The runners used by previous researchers w e r e generally m a d e of materials softer than mild steel. Usually, prolonged pre-test runs w e r e made t o establish good c o n t a c t so t h a t t h e b n g i - tudinal grooves on t h e runner would be well-matched with t h e ice s u r f a c e profile. Such surface conditions would be equivalent t o a smooth surface in t h e direction of t h e grooves. The e f f e c t s of t h e t h e r m a l conductivity observed a r e probably caused by such s u r f a c e conditions. The e f - f e c t s of hardness would mostly control t h e s u r f a c e abrasion in t h e c a s e of metallic runners.
Since t h e e l a s t i c modulus of ice is very low (less than 1/10 t h a t of most metals) t h e Hertzian c o n t a c t a r e a would b e l i t t l e affected.
References
(1) Bowden, F.P. and T.P. Hughes, Proc. Roy. Soc. A172 (1939) 280-297.
(2) Reynolds, O., Mem. Proc. l i a n c h e s t e r Lit. Phil. Soc. 431 (1898) 1-7.
(3) Tusima, K., J. Glaciol. 19 (1977) 225-235.
(4) Furushima, T., J. Japan. Soc. Snow Ice 34 (1972) 9-14.
(5) Evans, D.C.B., J.F. Nye and K.J. Cheeseman, Proc. Roy. Soc. London A347 (1976) 493-562.
