KNOWLEDGE

The accuracy of linear guideway includes the dimensional tolerance of height, width, and the running accuracy of the carriage on the rail. The standard of the dimension difference is built for two or more carriages on a rail or a number of rails are used on the same plane. The accuracy of linear guideway is divided into 5 classes, normal grade (N), high precision (H), precision (P), super precision (SP), and ultra precision (UP).

Running parallelism
The running accuracy is the deviation of parallelism between the reference surface of carriage and reference surface of rail when carriage moving over the entire length of rail.

Height difference (ΔH)
The height difference (ΔH) means the height difference among carriages installed on the same plane.

Width difference (ΔW2)
The width difference (ΔW2) means the width difference among carriages installed on a rail.

Note：When two or more linear guideways are used on the same plane, the tolerance of W2 and difference of ΔW2 is applicable to master rail only.
Note：The accuracy is measured at the center or central area of carriage.
Note：The rail is smoothly curved so that the required accuracy is easily achieved by pressing the rail to the reference surface of the machine. If it is mounted on a less rigid base such as an aluminum base, the curve of the rail will affect the accuracy of the machine. Therefore, it is necessary to deffne straightness of the rail in advance.

The accuracy grade for different applications shown as table below

Material and Hardness of PMI Ballscrews

The material of the sliding block and the shaft supporter is aluminum alloy, which has been treated with anodised and hard chrome plating, which has the effect of light weight, corrosion resistance and rust prevention. Compared with various types of slide rails, it has a price competitive advantage.

The accuracy of the Ball Spline is classiffed into three grades: normal grade (N), high accuracy grade (H) and precision grade (P), according to the runout of spline nut circumference in relation to the support of the spline shaft. shows measurement items.

Measurement item of the Ball Spline.：

Runout of the Spline Nut Circumference in Relation to the Support of the Spline Shaft

Perpendicularity of the Spline Shaft End Face in Relation to the Support of the Spline Shaft

Concentricity of the Part-mounting in Relation to the Support of the Spline Shaft

Straightness of the Flange-mounting Surface of the Spline Nut in Relation to the Support of the Spline Shaft

Preface
In recent years, more and more ballscrews are installed in various machines to meet the requirements of higher accuracy and better performance.
Ballscrews become one of the most widely used power transmission components. In CNC machines, ballscrews help improve their positioning accuracy and extend their service life. Ballscrews are also increasingly used to replace ACME screws in manually operated machines.
A ballscrew is normally preloaded to minimize the backlash of machine movement. Even a high precision ballscrew will not provide good accuracy and long service life if it is not installed properly.
This article discusses primary ballscrew problems and their precautions. Some measuring procedures are also discussed to help users locate the cause of an abnormal backlash.

The Cause and Precautions of Ballscrew Problems
Three major categories of ballscrew problems and their precautions are discussed as follows

Unsmooth operation
Defects from ballscrew manufacturing

• The return tube is not attached to the ball nut appropriately
• The track surface of the ballscrew spindle or the ball nut is too rough.
• The roundness of the ball nut or the screw shaft is out of tolerance.
• The lead or the pitch circle diameter of the ball nut / the shaft is out of tolerance.

Over-travel
Over-travel can damage the return tube and cause it to collapse or even break. When this happens, the steel balls will not circulate smoothly. They may break and damage the groove on the ball nut or the screw shaft under severe circumstances. Over-travel may happen during set-up or as the result of a limit switch failure or a machine collision. To prevent further damage, an over-traveled ballscrew should be checked or repaired by the manufacturer before it goes back to service.

Misalignment
Radial load exists if the center line of the ball nut's housing and the screw shaft's bearing support bracket are not aligned properly. The ballscrew unit may bend if this misalignment is too big. An abnormal wear may still happen even if the misalignment is not significant enough to cause a noticeable bending. The accuracy of a ballscrew unit will deteriorate rapidly if it is misaligned. The higher the preload is set in the nut, the more demanding the alignment accuracy is required in the ballscrew.

Foreign objects enter the ball path
Machined chips get in the ball track. The chips or dust generated during machining processes may be trapped in the ball track if wiper kits are not used to keep them away from the surface of the ballscrew unit. This may cause unsmooth operation, deteriorate accuracy and reduce service life.

Damaged return tube
The return tube may collapse and cause the same problems as mentioned above if it is hit heavily during installation.

The ball nut is not mounted properly on the nut housing
Eccentric load exists when the mounted ball nut is tilted or misaligned. If this is the case, the motor current may fluctuate during rotation.

Ballscrew unit is damaged during transportation

• During installation, avoid nuts separating away from screw, otherwise the balls will getting out of the nut, that lead to change of the preload and damage of the circulation system and wiper.
• Due to the low friction coefficient, nuts will fall down because of its self weight during vertical deposition; this kind of damage should be avoided, once happened,it should be inspected by manufacturer preventing further damage

Too much gap
No preload or insufficient preload
The ball nut will rotate and move downward by its self weight when a non-preloaded ballscrew is held vertically with the screw shaft constrained. A significant backlash may exist in a non-preloaded ballscrew unit. Therefore non-preload ballscrews are only used in the machinery, where operation resistance but not positioning accuracy low is the major concern.
PMI can determine the correct amount of preload based on different applications. We can also preset the amount of preload before shipment. Be sure to clearly specify the operation condition of your application when you order a ballscrew unit.

Inappropriate bearing selection and installation

• Angular ball bearings should be used in ballscrew installation. A ball bearing with high pressure Service Problems Analysis of Ball Screws A1-263angle specially designed for ballscrew installation is even a better choice. A regular deep groove ball bearing will generate a signiffcant amount of axial play when axially loaded. It should not be used in this application.
• Two lock nuts and a spring washer should be used in the bearing installation to prevent them from getting loose in operation.
• The perpendicularity between the bearing seating face and the thread axis of the bearing locknut on the ballscrew, or the parallelism between the opposite faces of the locknut is out of tolerance causing the bearing to tilt. The thread for bearing lock nut and the seating face of a bearing in the ballscrew journal should be machined in one setting to ensure the perpendicularity. It is even better if they can be ground.
• If the bearing is not attached to the screw shaft properly, it would cause axial play under load. This problem may be caused by the bearing journal of the screw shaft being too long or the nonthreaded part of the screw shaft being too short. To solve this problem used the collar.

Parallelism or flatness of the housing surface is out of tolerance
In a machine assembly, a shim bar is frequently located between the housing location surface and the machine body for adjustment purpose. The clearance of table movement may vary at different locations if the parallelism or atness of any matching component is out of tolerance regardless of whether they are ground or scraped.

The ball nut housing or the bearing housing is not rigid enough
The ball-nut-mounted housing or the bearing-mounted housing may deflect under components' weight or machining load if it is not rigid enough.

The ball nut housing or the bearing housing is not mounted properly

• Ball-nut-seated screws become loose due to vibration and lack of a spring washer.
• Ball-nut-seated screws are not seated ffrmly because the screws are too long or the thread holes on housing are too short.
• Components may become loose due to vibration or lack of locating pin(s). Solid pins instead of spring pins should be used for location purposes.
• Not enough locking forces for ffxing screw because of too short screw.s

The motor and the ballscrew spindle are not assembled properly

• There will be a relative rotation between the motor shaft and the ballscrew spindle if the connecting coupling is not installed ffrmly or the coupling itself is not rigid enough.
• Key is loose in the groove. Any inappropriate match among the hub, key, and key seat may cause these components to generate backlash.
• Driving gears are not engaged properly or the driving mechanism is not rigid. A timing belt should be used to prevent slipping if the ballscrew is to be driven by a belt.

Fracture
Broken bearing ball
Cr-Mo steel is the most commonly used material for bearing balls. It takes about 1,400kg (3,080lb) to 1,600kg (3,520lb) to break a steel ball of 3.175 mm (1/8 in) diameter. The temperature of an underlubricated or non-lubricated ballscrew raises substantially during operation. This temperature raise could make the bearing balls brittle or break which cause damage to the grooves of the ball nut or the ballscrew spindle consequently.
Therefore, lubricant replenishment should be considered during the design process. If an automatic lubricating system is not available, periodical grease replenishment should be scheduled as part of maintenance program

Collapsed or broken return tube
Over-travel of the ball nut or an impact on the return tube could cause the return tube to collapse or break. This may block the path of bearing balls and cause them to slide instead of rolling and break eventually

Ballscrew shaft end breaks

• Inappropriate design: Sharp corners on the ballscrew spindle should be avoided to reduce local stress concentration.
• Bend of screw shaft journal: The seating surface of the bearing of the ballscrew and the thread axis of the bearing's lock nut are not perpendicular to each other or the opposite sides of the lock nut are not parallel to each other. This will cause the end of the screw shaft to bend and eventually break. The amount of deflection at the end of the ballscrew shaft before and after the bearing's lock nut being tightened should not exceed 0.01 mm (0.0004 in).
• Radial force or fluctuating stress: Misalignment in the ballscrew installation creates abnormal fluctuating shear stress and causes the ballscrew to fail prematurely.
• It should be avoided, so that the dimension of the ball screw shaft end doesn't differ too much from the designed ball screw shaft section area.

Influence of temperature raise on ball screw
During the operation of ball screws, the accuracy of machine drive system will influenced by the raise of the temperature, especially for the high speed and high accuracy machines. Following factors affect the temperature raise of ball screws.

• The influence of Preload
  Increase the rigidity of ball screw nut in order to avoid the lost motion of the machine drive system, that means increase the preload of the nut to a certain standard. Once the nut being preloaded, the friction torque will be increase, making the temperature raised during operation. PMI recommended that the preload force should be 1/3 of the maximal axial load and is not bigger than 10% of the dynamic load, in order to obtain the optimal life time and lower temperature raise effect.
• The Inuence of Pretension
  The elongation and deformation of ball screws because of heat will deteriorate the position accuracy. The amount of thermal elongation can be calculated by certain formula and cpmpensated by preloading torque. The target value of the Pretension compensation is the negativ T value on the diagram. Too much pretension will burn the support bearing. Therefor PMI recommended that the pretension should be smaller then the pretension by 5°C; however when the ball screws diameter is over 50mm, it is not suitable for a preloading torque, that means large Pretension forces will be needed when the diameter is large and will burn down the support bearing. PMI recommened, that 2~5°C of temperature raise should be used as standard to compensate the value T (about -0.02~-0.06mm every 1000mm of ball screw)
• The Inuence of Lubrication
  The choice of the lubrication will directly affect the raise in temperature of the ball screws. The ball screws of PMI should be lubricated by oil or grease. Normally lubrication oil for bearings will be recommended as ball screw lubrication, and grease from lithium soap will be recommended as lubrication grease. The choice of viscosity of the lubrication should be according to the operation speed, the working temperature, and the situation of load.

Low viscosity lubrication should be choosed during high speed and low load situation; high viscosity lubrication during low speed and high load situation. Normally, viscosity range of lubrication will be recommended at 32~68cSt (ISO VG 32~68)(DIN51519) during 40°C, high speed; viscosity range of lubrication will be recommended over 90cSt(ISO VG 90) during 40°C, low speed. By application of high speed and heavy load, force cooling must be used in order to reduce the temperature, and using hollow ball screw or cooling oil though nut to meet the cooling consequent.