How to know the lead screw is suitable for your application?

●Load capacity of lead screw

Plastic nuts are suitable for light loads of less than 100 pounds,although plastic nuts of 300 pounds and up can be designed.Bronze nuts,on the other hand,can be used in applications over thousands of pounds.If the load requirements exceed the lead screw capacity,ball screw is a better choice.

●Lead screw efficiency

The efficiency of ball screws is relatively constant,usually better than 90%.The efficiency of the lead screw is usually between 20% and 80%,highly dependent on the value of the helix angle.Helix angle is the inverse cut to pitch diameter of the lead.It is the angle of advance of the thread.Generally speaking,higher helix angle means higher efficiency,because the less energy that drives the lead screw is used to overcome friction.This is because on high helix lead,the number of times the screw must rotate to obtain a given linear displacement is lower.One disadvantage of high helix angle is that more torque is required to turn the screw.

●Lead screw speed

Ball screws typically employ moderate leads between 0.200 and 0.500 in./rev,despite the presence of high-helix products.Lead screws have a variety of leads from as little as 0.050 to 2.00 in./rev or higher.Using a variety of leads can provide up to 70 "/s change in jog speed.This lead screw capability can provide advantages in many applications.For example,a device that must accurately position a payload can use a lead screw with a low helix angle for high positioning resolution.Other applications benefit from fast jog speeds and low screw speeds,providing quiet operation and long life.The maximum speed of the lead screw is limited by the critical speed of the lead screw(the speed at which resonance occurs).Lead screw nuts can be driven at very high RPM,but depending on the load applied,heat buildup may occur,limiting duty ratio.

The payload can reverse drive the ball bearing screw,forcing the nut to rotate the screw due to inherent efficiency of 90% or more.

●Lead screw duty cycle

Plastic and stainless steel lead screw assemblies are usually limited to a duty cycle of 50% at rated load.Lead screw assemblies using bronze nuts have a higher load capacity,but these heavier loads increase the heat of friction,so their duty cycle must be lower,often as low as 10%.This can be calculated more accurately using the PV relationship.The lead screw may operate at 100% duty cycle at light loads and moderate speeds,or at lower loads at high loads and low speeds,or at low loads and high speeds,but not both.High loads and high speed relative to a given thread size and nut design can cause overheating and failure.

●Lead screw backlash

Backlash is an important consideration in applications where positioning accuracy is very important.Standard ball screw and lead screw configurations typically have a clearance of 0.002 to 0.010 in.Anti-backlaash lead nuts eliminate free clearance and improve repeatability.Most designs place a compression spring or other compliant member between the two nut halves to eliminate radial clearance.The preload between the nut halves must equal or exceed the axial load applied in the direction of the loading assembly through the coiler to prevent loss of motion.The result is higher torque requirements,often requiring larger motors.

Some proprietary anti-backlash nuts eliminate free clearance between nuts and screws without introducing excessive drag torque.Cam surfaces on a biasing spacer let it rotate and translate relative to the mating nut halves,accommodating clearances from wear of the nut threads.The angle of the CAM surface allows the two parts to self-lock,preventing a ramp drop or reverse clearance under load.A torsion spring in the CAM between the half nuts provides sufficient torque for the nut bias.Only when the wear at the nut thread junction creates a gap does the CAM advance to eliminate the gap.The self-locking action prevents CAM movement except during load reversal.

Standard ball screw and screw configurations typically have a clearance of 0.002 to 0.010 in.Anti-recoil lead nuts eliminate free clearance and improve repeatability.Lead screw noise Ball screw is prone to produce a certain amount of noise due to the recirculation of ball bearings.The lead screw,on the other hand,is quiet.Although in the absence of lubrication,the friction impedance from sliding can produce a flutter or screech.

●Lead screw corrosion resistance

Ball screws made of carbon steel(a common choice)may be sensitive to some corrosive environments.On the other hand,lead screws with stainless steel screws and plastic nuts may be highly resistant to corrosion.However,plastic nuts may be limited to temperatures between 30 and 150°F.Lead screws with stainless steel screws and bronze nuts also have good corrosion resistance.

●Life calculation of lead screw

The life of the lead screw can be nonlinear because of friction and wear.A single wear coefficient is generally not sufficient to predict the performance of a lead screw assembly throughout its life cycle.Life expectancy was estimated using case studies under controlled operating conditions.Failure is predictable and can occur rapidly when the combination of load,speed,and duty ratio exceeds certain limits.For applications that run at speeds greater than 500 rpm near the recommended design load,consult your lead screw supplier's application engineering department.They can calculate the pressure velocity(PV)factor,which is a key design factor in the correct selection and selection of lead screw assemblies using polymer nuts.

●The final description of the pressure - velocity factor 

PV coefficientIs the product of the pressure and velocity between the nut and the screw.It helps determine the load,speed,and duty cycle the nut can handle.Plastic materials have an inherent PV rating,which is the point at which friction heat causes the plastic to deform permanently.Therefore,the greater the load placed on the lead screw assembly,the slower it must be turned to avoid exceeding the PV limit of the nut.Similarly,the faster the nut turns,the lower the available load capacity.The main failure modes of plastic nuts are wear and PV.Wear failures occur more slowly than photovoltaic failures and can be explained by applying case studies and testing.

● Lead screw lubrication

All ball screws require lubrication to provide reasonable life.Lead screw mechanisms using bronze nuts also require lubricant,usually in the form of thick damping grease.In contrast,a lead screw assembly with a plastic nut can function well without lubricant due to the internal lubricant in the nut material,but the use of a gel type lubricant increases the allowable load and extends the service life by reducing friction between the nut and screw.For moderate loads,speeds,and cycles in an office environment,lubrication should last 500,000 to 2,500,000 inches.Bad environment will shorten the service life of grease.

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