Prony bridle

Scheme of a Prony bridle
M = torque, F = weight force, l = lever length

The Prony bridle (named after the French engineer Gaspard de Prony 1755–1839) is a device for measuring torques on rotating shafts. Other names are wood brake or shoe brake . Together with the water vortex brake, the eddy current brake and the generator brake, it belongs to the brake dynamometers .

The Prony bridle essentially consists of a lever with adjustable jaws encompassing a friction wheel at one end and a weighing pan at the other end, and is clamped with the jaws on the friction wheel or the rotating shaft whose torque is to be measured. Masses placed on the weighing pan prevent the bridle from turning with the horizontal lever. Your weight represents the measured torque in the product with the lever length.

The torque generated and measured by the machine under investigation is set by changing the pressure of the brake shoes on the shaft (the wheel) using the adjusting screws (see illustration). The shaft rotates at a constant speed. In order to bring the bridle into the measuring state (the lever arm does not touch the stops), the weighing masses are changed. From the distance between the center of the brake shoe and the point of application of the weight , the torque delivered by the machine at the specified speed results with: ${\ displaystyle l}$ ${\ displaystyle F}$

{\ displaystyle \ mathrm {M} = l \ cdot \ mathrm {F}}

Since the bridle is difficult to keep in balance due to the naturally inconsistent frictional torque, the Prony bridle is usually only used to determine the starting torque of electric motors. Here the electric motor is braked almost to a standstill.

The mechanical performance is calculated from the quotient of the work performed and the time required for it . If the duration of a full revolution is the speed , and the work done is the product of the force and the length of the path, i.e. the circumference of the circle , that is ${\ displaystyle \ Delta \ mathrm {W}}$ ${\ displaystyle \ Delta t}$ ${\ displaystyle \ Delta t}$ ${\ displaystyle \ textstyle n = {\ frac {1} {\ Delta t}}}$ ${\ displaystyle F}$ ${\ displaystyle 2 \ pi \ cdot l}$

{\ displaystyle \ mathrm {P} = {\ frac {\ Delta \ mathrm {W}} {\ Delta t}} = {\ frac {2 \ pi \ cdot \ mathrm {M}} {\ Delta t}} = 2 \ pi \ cdot \ mathrm {M} \ cdot n}

.

Notes and individual references

↑ Brake dynamometers must B. from a prime mover "absorb power and measure the torque at the same time". Cf. HÜTTE , Theoretical Basics, 28th edition, 1955, page 1453
↑ In the case of prime movers with decreasing torque when the revs increase, a constant revolutions per minute is set at a given load (e.g. with electric DC motors ), with other prime movers (e.g. internal combustion engines ) the revs must be kept constant by a special measure.
↑ D. Meschede: Gerthsen Physik , Springer-Verlag (2010), 24th revised edition, ISBN 978-3-642-12893-6 , Section 1.9.4: The technical meaning of friction
↑ It is better if the brake shoes are pressed on by means of pretensioned springs.

[1] Brake dynamometers must B. from a prime mover "absorb power and measure the torque at the same time". Cf. HÜTTE , Theoretical Basics, 28th edition, 1955, page 1453

[2] In the case of prime movers with decreasing torque when the revs increase, a constant revolutions per minute is set at a given load (e.g. with electric DC motors ), with other prime movers (e.g. internal combustion engines ) the revs must be kept constant by a special measure.

[3] D. Meschede: Gerthsen Physik , Springer-Verlag (2010), 24th revised edition, ISBN 978-3-642-12893-6 , Section 1.9.4: The technical meaning of friction

[4] It is better if the brake shoes are pressed on by means of pretensioned springs.