Exp 5 Skid Resistance

EXPERIMENT NO-5

MEASURING SURFACE FRICTIONAL PROPERTIES USING BRITISH PENDULUM SKID RESISTANCE TESTER.

RELATED THEORY Skid Resistance

The British Pendulum Skid Resistance Tester is an impact type pendulum used to measure the energy loss when a rubber slider edge is propelled over a test surface. The values measured represent the frictional properties and are expressed as BPN (British Pendulum Number).

SKIDDING Skid Resistance

In skidding phenomenon, the distance travelled by wheel on the road is more than the circumferential movement.When brakes are applied, wheels get locked and still if the vehicle moves ahead, longitudinal skidding is said to occur. Skid Resistance

SLIPPING

In slipping, the distance traveled by wheel on the road is less than the circumferential movement. If the wheels of the vehicle are rotating but the vehicle is not moving forward, then slipping is said to occur. Skid Resistance

 MEASUREMENT OF TEXTURE DEPTH

 It is also necessary to record the surface texture of the road surface tested. This is measured by Sand Patch Method. A known volume of fine sand of uniform particle size is poured on the road and spread to form a patch of a regular shape so that its area couldn’t be measured. Sand should be spread in such a manner that all “valleys” are filled to level of “peaks”. Then the texture depth can be computed by using the formula;

TEXTURE DEPTH =  VOLUME OF SAND / AREA OF PATCH

ROAD SURFACE TEXTURE	TEXTURE DEPTH
Coarse/Open Texture	≥ 0.02”
Medium Texture	0.02” –  0.01”
Fine Texture	≤ 0.01”

More the texture depth, better is the road surface but a high value of texture depth also increases the chances of severe injuries. Skid Resistance

SCOPE & SIGNIFICANCE

Friction between vehicle and road surface plays a vital role in determining the

•  maximum operating speed and the distance required in safely stopping the vehicles.

• Road pavements are designed to provide reasonably high coefficient of friction but with the movement of traffic, the aggregates in the wearing coarse of the road get polished resulting in reduction of the frictional or skid resistance of the road. Skid Resistance

• British Pendulum Tester is used to measure the frictional resistance of road at sections which appear to be potentially slippery and unsafe against skidding.

• British Pendulum Tester is used for field and laboratory tests.

FACTORS AFFECTING SKIDDING RESISTANCE

• Condition of Tires – Vehicles having smooth tires would have slightly lower skidding resistance.

• Effect of Road Temperature – Skidding resistance of wet roads is higher in winter than in summer. Skidding resistance of roads is at its lowest between April and September. So roads with satisfactory values in winter may prove slippery during summer. Skid Resistance

• Effect of Water – Skidding resistance is lower on wet than on dry pavements. Skid Resistance

• Ø Type of aggregate – sand stones and granites are more resistant to polishing action than limestone.

Higher the skid-resistance results, better is the road surface but a high value of skid-resistance also increases the chances of severe injuries.

APPARATUS

(a)       Parts

•   Slider – Slider assembly consists of an aluminum backing plate to which a rubber strip 3’’ x 1’’ x ¼’’ is bonded.
• Leveling screws
• Scale
• Drag pointer
• Locking and control knobs
• Frictional rings

(b) Accessories

• Contact path gauge – shall consist of a thin ruler suitably marked for measuring contact path length between (4 7/8’’ and 5’’).
• Water container
• Surface thermometer and
• Brush

REPARATION OF APPARATUS

1. Leveling – apparatus is levelled using 3 levelling screws until the bubble is centered in the spirit level.

1. Zero adjustment – raise the pendulum mechanism by loosening locking knob, till it swings free of test surface. Tighten the knob. Place pendulum in release position and drag pointer in horizontal position and drag pointer in horizontal position. Execute a free swing and note the pointer reading. If reading is not zero; make adjustment by friction ring until pendulum swing carries the pointer to zero. Skid Resistance

1. Slide length adjustment – with pendulum hanging free, place spacer under adjusting screw of lifting handle and allow pendulum to move slowly to the left until the edge of the slider touches the surface. Place gauge beside slider and parallel to the direction of swing to verify the length of contact path which should be between (4 7/8’’ and 5’’) on flat surfaces as measured from trailing edge to trailing edge of the rubber slider. Skid Resistance

Place pendulum in release position and rotate drag pointer counter clockwise until it comes to original position.
Inspect the road and choose the section to be tested. Set the apparatus so that the slider swings in the direction of traffic.

TEST PROCEDURE

Ø Apply sufficient water to cover the test area thoroughly. Execute one swing but don’t record reading.

• Ø Make four more swings, rewetting the test area each time and record the results. The readings should not differ by more than 3 units. Skid Resistance
• Report the individual values as B.P.N. also note down the age, condition, texture and location of test area.

CATEGORY	TYPE OF SITE	‘SKID RESISTANCE’ ON WET SURFACE	STANDARD OF SKIDDING RESISTANCE REPRESENTED
A	Most difficult sites such as;	Above 65	‘Good’: fulfilling the requirements even of fast traffic, and making it most unlikely that the road will be the scene of repeated accidents.
	i)-      Roundabouts
	ii)-      Bends with radius less than 500 ft on derestricted roads.
	iii)-      Gradients, 1 in 20 or steeper, of length greater than 100 yd.
	iv)-      Approach to traffic lights on derestricted roads.
B*	General requirements, i.e. roads and conditions not covered by categories A & C.	Above 55	‘Generally Satisfactory’: meeting all but the most difficult conditions encountered on the roads.
C*	Easy sites, e.g. straight roads, with easy gradients and curves, without junctions, and free from any features, such as mixed traffic, especially liable to create conditions of emergency.	Above 45	‘Satisfactory only in favorable circumstances.’
D	All sites	Below 45	‘Potentially Slippery’
* On smooth-looking or fine-textured roads in these categories, vehicles having smooth tyres may not find skid-resistance adequate. For such road accident studies should also be made to ensure that there are no indications of difficulties due to skidding under wet conditions.

OBSERVATIONS & CALCULATIONS

TEXTURE DEPTH Skid Resistance

Group	Dimensions of Patch      (cm x cm)	AREA (cm2)	Volume of Sand            (ml = cm3)	Texture Depth
				(cm)	(inches)
1	33.44 x 17.52	586.1	50	0.0853	0.033
2	34 x 21.5	731	50	0.0684	0.0269

BRITISH PENDULUM NUMBER

Group	Date	Distance from	Distance (m)	Surface Texture (in)	Skid-Resistance (BPN)	Mean (BPN)
1	10/6/2010	Triangle	0.856	0.033	70,64,64,66	64.67
2	10/6/2010	Footpath	1.33	0.0269	63,60,62,60	60.667

COMMENTS

After performing the experiment and calculating the surface texture it is concluded that the road surface texture on which we performed the experiment was Coarse/Open Textured surface for both the groups & the standard of skid resistance for 1^st group came out to be Good but for second group it came out to be Generally Satisfactory.




Skid Resistance

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