What is the difference between backlash and hysteresis in linear systems?

In linear systems,backlash and hysteresis are often referred to as the same phenomenon.But while they all cause loss of motion,they do so for different reasons and in different ways.

Backlash is caused by clearance or play,between mating parts,and dead bands are introduced when the direction of travel is reversed.In a dead band,no movement occurs until the clearance between mating parts are eliminated.

Common components that encounter clearance include ball screws,lead screws,pulley systems,and gears.In recirculating bearing systems,the application of preload can reduce or eliminate backlash by removing the clearance between the ball(or roller)and the raceway.Some non-recirculating systems use alternative methods,such as springs or specially designed lead screw nuts,to reduce or eliminate clearance.

Axial play,or backlash,is caused by clearance between mating components.

Although backlash is often seen as a negative feature of mechanical systems,it is not always harmful.First,it is expensive and,in most cases,impractical to produce completely backlash-free components.Methods of reducing clearance inevitably increase friction and wear.If some recoil can be tolerated in the application,the available components will be cheaper,more readily available,and in many cases will have a longer useful life.In gears and gearboxes,some clearance is needed to engage the gears without putting too much pressure on the teeth and increasing friction.

Backlash is necessary for proper engagement of gear teeth.

Hysteresis is usually associated with magnetic systems and manifests as hysteresis loss in motors.Simply put,hysteresis is the relationship between the material's response to the initial load(or magnetizing force)and the material's recovery after the load(or magnetizing force)is removed.For example,when iron is magnetized by an external field,the magnetization of iron lags behind the magnetization force.When the magnetization force is removed,iron retains a certain amount of magnetism.In other words,iron will not fully return to its unmagnetized state unless the opposite magnetizing force is applied.

The hysteresis loop shows how the induced magnetism of a ferrous material lags behind the applied magnetic field.Inelastic materials react similarly when a force is applied and removed.

In mechanical systems,hysteresis is related to the elasticity of the material.For example,when the steel balls in a ballnut are moved from a non-load-bearing zone to a load-bearing zone,the force they are subjected to increases,causing them to deform slightly.But because of the elastic properties of steel,when the ball bearings are moved back into the non-load-bearing area of the nut,they do not fully return to their original shape.This persistent microscopic deformation is due to hysteresis.

Hysteresis also affects the behavior of the drive shaft in a mechanical system.When torque(torsional force)is applied to the shaft,it creates internal stress and causes the shaft to change shape.This change in shape is called strain(or torsional strain in the case of torsional loads).In perfectly elastic materials,the relationship between stress and strain is linear.But few materials are completely elastic,and the inelasticity of materials gives them nonlinear stress-strain curves.This non-linear behavior as the force increases and decreases is called hysteresis.

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