Slip

Slip (from "slipping") generally refers to the deviation of the speeds of mechanical elements or fluids in frictional contact with one another under tangential loading.

Belt drives

Two pulleys connected by a drive belt ( round belt or flat belt ) should rotate at the same peripheral speed, namely the speed of the belt. If, however, power is transmitted, the part of the belt that runs towards the driving pulley ( load strand ) is stretched compared to the part returning with less tensile force ( slack strand ) and therefore runs at greater speed. These two speeds are approximately the peripheral speeds of the disks. This so-called elongation slip is proportional to the elongation and thus almost proportional to the force transmitted.

The change in elongation occurs under sliding friction on the pulleys when the belt runs off and in an area before that, the extent of which depends on the belt tension.

In addition to the expansion slip, sliding slip can also occur in the belt drive, mainly briefly when starting, but also over the entire contact surface in the event of overload. Then the belt wears out quickly. Slippage can occur in the input or output gear - typically in the wheel with the smaller wrap. In extreme cases, the output stops.

The sliding slip can be reduced structurally by increasing the wrap angle, by using wedge (ribbed) belts and by using a belt material with a higher coefficient of friction. In addition, belt drives usually have a tensioning device that postpones the occurrence of sliding. Most of the time, a belt drive just needs to be tensioned in order to return from sliding slip to stretching slip. Retensioning is necessary when the slippage shows up through belt squeaking.

To avoid slippage entirely, toothed belts or chains are used. The basically other alternative are gear drives and (for the function of spatial power transmission ) rotating shafts .

Hydraulic clutch

Slip also occurs in hydraulic clutches. The power transmission is achieved through the viscosity of the clutch fluid. If the viscosity drops sharply due to heating due to overloading, the viscous slip also changes into a residual slip that is no longer technically usable. A non-linear abrupt transition from proper function to faulty function does not occur as a result of the transition from static friction to sliding friction, but when a limit temperature is exceeded or after the clutch fluid has aged.

Three-phase asynchronous machine

The slip is the difference in speed between the stator rotating field (stator) and rotor (rotor), usually given as a percentage value related to the rotating field speed.

If the rotor were to rotate at the same speed as the stator rotating field, no change in magnetic flux would be possible in the rotor and the rotor would not generate any torque. The rotor speed is therefore always lower than the rotating field speed in motor operation . Example: In a three-phase asynchronous machine with a stator coil for each of the three phases, the rotating magnetic field rotates at a mains frequency of 50 Hz at 3000 revolutions per minute. According to the nameplate, the speed of the armature is only 2700 rpm. The slip of 300 rpm is load-dependent and is almost proportional to the rotor efficiency. At rated motor power, it is between 1.2% and 10% of the rotating field speed , depending on the motor size. Smaller three-phase motors have poorer efficiencies and consequently also the larger slip values:

{\ displaystyle s = {\ frac {n _ {\ mathrm {D}} -n_ {2}} {n _ {\ mathrm {D}}}},}

with = rotating field speed and = rotor speed ${\ displaystyle n _ {\ mathrm {D}}}$ ${\ displaystyle n_ {2}}$

propeller

Technically known as “slip”, the slip of a ship's propeller is the difference between the theoretically covered distance and the actual distance covered, relative to the theoretical distance covered.

The technical value of a propeller is determined in pitch . A propeller with a pitch of e.g. B. 4.80 m and 120 revolutions per minute, theoretically covers a distance of:

{\ displaystyle 4 {,} 80 \, \ mathrm {m} \ times 120 / \ mathrm {min} \ times 60 \, \ mathrm {min} = 34 {,} 56 \, \ mathrm {km}}

In nautical miles :

{\ displaystyle {\ frac {34 {,} 56 \, \ mathrm {km}} {1852 \, \ mathrm {m / sm}}} \ approx 18 {,} 66 \, \ mathrm {sm}}

In 24 hours: approx.

{\ displaystyle 447 {,} 9 \, \ mathrm {sm}.}

The propeller can theoretically cover around 447.9 nm in 24 hours. This value is usually higher than the distance actually covered, which was calculated by the nautical ship's command. This then results in a positive “slip” value. In contrast, if the distance actually covered is higher, the result is a negative value for the "slip". The latter case in the example: Is the nautically calculated "true" distance covered e.g. B. 470 nm, this results in a slip of:

{\ displaystyle {\ frac {447 {,} 90-470} {447 {,} 90}} \ approx -4 {,} 9 \, \%.}

This value says a lot about the condition of the underwater hull. The water flow and wind conditions have no influence when using a log that measures the travel through water (FdW).

Since the speed of a ship's engine never stays constant at sea, the stroke counter (revolution counter) is entered in the engine log every day at 12:00. From this you can easily calculate the total speed of the last 24 hours. If you stick to the exemplary values, this results in a total of 172,800 revolutions in 24 hours.

This total speed, multiplied by the gradient and divided by 1852 m / nm, would result in a distance of about 447.9 nautical miles.

A negative slip can occur in strong aft winds and strong currents .

wheel

The slip is the ratio of the rotational speed of a driven wheel to that of a (hypothetical) non-driven and thus a form-locking idler wheel : . ${\ displaystyle \ omega}$ ${\ displaystyle \ omega _ {0}}$ ${\ displaystyle S = {\ frac {\ omega - \ omega _ {0}} {\ omega _ {0}}}}$

As soon as drive or braking forces are transmitted to the wheel, a low slip value that differs from zero occurs. If the wheel is driven or braked more strongly than the frictional connection limit allows, the slip increases until uncontrolled spinning ( skidding ) or slipping / locking (gliding) of the wheels can occur (railway terminology in brackets). The slip value is then 0 or 0; According to this definition, whether there is drive or brake slip can be recognized by the sign of the slip. The relationship between force related to a constant wheel load and slip is referred to as the μ-slip curve. ${\ displaystyle S}$ ${\ displaystyle \ gg}$ ${\ displaystyle \ ll}$

Transmits a wheel in addition cornering forces , so a resulting total force equal to the adhesion limit leads to a correspondingly high slip in the direction of the total force, causing a reduced cornering (see circle of forces ): Therefore, a front-wheel drive motor vehicle tends during acceleration in curves to understeer , a rear-wheel drive to oversteer . For railway vehicles, it comes in such a situation to a sideways displacement of the wheel and possibly a start of the flanges to the outside of the curve rail head .

In addition, there is an outdated and, from a physical point of view, less consistent definition that differs for the "driving" and "braking" states:

Driving or drive slip: and brake slip: ${\ displaystyle S_ {T} = {\ frac {\ omega - \ omega _ {0}} {\ omega}}}$ ${\ displaystyle S_ {B} = {\ frac {\ omega _ {0} - \ omega} {\ omega _ {0}}}.}$

If this definition is used, different slip stiffnesses result for the driving and braking states, but these have advantages when calculating with the slip: On the one hand, it is always positive and, on the other hand, singularities in the solution are avoided that occur when dividing by zero arise. The definition of traction slip applied to a locking wheel would not provide a sensible solution during braking.

The mechanism of force generation can be explained with the brush model. Between the carcass and the road there is the elastic tread, the tread particles of which experience tension. The tread particles run into the mountain pass without tension and are increasingly deformed by the difference in speed between the carcass and the road.

Screws

In the case of screw connections, what is known as "slip" or "hole play" is the distance that would have to be overcome before a (normally undesirable) force transmission would take place between the screw shaft and the borehole wall; the shaft diameter of the screw (for St screws) and the drill hole diameter are not the same. Normally, such a slip does not take place, as the forces to be transmitted are prevented by the pressure of the screw (s) between the components and the associated friction - this is the design goal of a screw connection.

literature

Günter Springer: Expertise in electrical engineering. 18th edition, Verlag - Europa-Lehrmittel, 1989, ISBN 3-8085-3018-9 .
Karl-Heinz Dietsche, Thomas Jäger: Kraftfahrtechnisches Taschenbuch. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3-528-23876-3 .
H. Yamazaki, M. Nagai, T. Kamada: A study of adhesion force model for wheel slip prevention control . In: JSME International Journal Series C . tape 47 , no. 2 , 2004, p. 496–501 , doi : 10.1299 / jsmec.47.496 (model and experiment on wheel-rail contact).

Web links

Wiktionary: slip - explanations of meanings, word origins, synonyms, translations

Individual evidence

↑ http://www.fast.kit.edu/lff/1011_2261.php
↑ Günter Leister: Vehicle tires and chassis development: strategy, methods, tools . Vieweg + Teubner, 2009, ISBN 978-3-8348-0671-0 , p. 109 ( limited preview in Google Book search).

[Büstenmodell-1] ttp://www.fast.kit.edu/lff/1011_2261.php

[2] Günter Leister: Vehicle tires and chassis development: strategy, methods, tools . Vieweg + Teubner, 2009, ISBN 978-3-8348-0671-0 , p. 109 ( limited preview in Google Book search).