How to measure friction? Method 1 Weight ratio

The sketch shown originates from Leonardo da Vinci (ca 1500). He studied friction by measuring the load hanging on a cord, at which the block begins to slide. The coefficient of friction is found by the quotient of the dead weight of the mass hanging on the cord and the mass of the block, i.e.
µ =  Ff / N = mdead weight / mblock

Static coefficient of friction - dynamic coefficient of friction The moment at which the block begins to slide (break away force) is the so called static friction, the force at which the block continues to slide is the dynamic or kinetic coefficient of friction. For most material combinations the value of the static friction exceeds that of the dynamic friction. Be aware that the dynamic friction can still be dependent on velocity, contact pressure, temperature and surface roughness. The static friction can be dependent on the time that the block is in rest, which is typically the case when lubricated. Method 2:
Spring balance

Pull a spring balance connected to the block  and slowly increase the force until the block begins to slide. Make sure the spring balance is parallel to the surface. The reading on the spring balance scale when the load begins to slide is a measure for the static friction, while the reading when the block continues to slide is a measure of dynamic friction. The coefficient of friction is simply µ =  Fspring /Fnormal =  Fspring /(mblock ·g ), g=9.81 m/s˛

Hint: Pulse rotation sensors (multi-turn potentiometers, pulse encoders) often prove to be very useful to create low cost sensors for measuring displacement by combining the sensor with a cable and a pulley, for measuring torque with a torsional spring, for measuring force with a wire, a pulley and a spring etc. Method 3: Tilted plane

Place a block on a tilted plane and increase the angle of tilt until the block begins to slide. The tangent of the tilting angle just found is the so called "friction angle". This angle is related to the coefficient of friction µ, i.e. µ = tanř = Ff /F

 from Angle of tilt in degrees Coefficient of friction to Angle of tilt in degrees Coefficient of Friction Slope
Method 4: Clamping

To measure the static coefficient of friction under conditions of high contact pressure the object may be clamped between two surfaces. The force necessary to put the object in motion must be halved to obtain the friction force because of the two contacting surfaces.

Method 5: Pendulum The pendulum is suitable to analyze the static and dynamic friction under reciprocal motion by monitoring the bearing torque. This however requires a torque sensor. The energy loss of combined static and dynamic friction can be analyzed by considering the reduction of the amplitude of motion in time. This only requires a simple rotary potentiometer or pulse rotation sensors to visualize the amplitude reduction in time.

Method 6: Motorized Tribometers

In the measuring methods discussed above the friction coefficient is measured in fresh contacts, not after running in. The coefficient of friction may change significantly during first half hour of sliding. The time necessary to obtain a stable value of the coefficient of friction can be observed in a motorized tribometer by monitoring the friction over time. This method is common for measuring the specific wear rate and the contact temperature during operation. You may visit the useful links on the right of this window to find more information about motorized tribometers.

www.tribology-abc.com