friction guideways

8/10/2019 Friction Guideways

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CNC Hardware

Friction guideways:

Vee or Inverted Vee Guideways:

These are widely used on machine tools, especially on lathe beds.

Advantages of this is that parallel alignment-of the guideway withthe spindle axis is not affected by wear. There is a closing actionas the upper member settles on the lower member, and thisautomatically maintains the alignment.

Jibs are, therefore, not required with the vee guideway to take upthe clearance caused by the wear.


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Flat and Dovetail Guideways:

o Flat or dovetail forms are used on CNC machine tools.

o The flat guideways have better load- bearing capabilities than theother guideways.

o After a period of use, wearing may occur owing to the sliding of

the surfaces over each other. Jibs are used to ensure accuratefitting of the slide to both the flat and dovetail guideways.

o The jibs are tapered and can be adjusted to reduce excessiveclearance caused by wear.


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o The metal-to-metal contact on the vee, flat and dovetail types ofguideway is normally cast iron to cast iron.

o The cast iron may be heat treated to increase its hardness, andthe surfaces ground to obtain the required accuracy.

Cylindrical guideways:

o In cylindrical guideways, the bore in the carriage housing providessupport all around the guideway.

o For relatively short traverses and light loads, cylindrical guidewaysare very efficient.

o A limitation on the use of these guideways for long traverses isthat if the guide bar is supported only at each end, it may sag orbend in the centre of the span under a load.

Antifriction Linear Motion (LM) Guideways:

Antifriction linear motion guideways are used on CNC machine tools to:

(a) Reduce the amount of wear


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(b) Improve the smoothness of the movement

(c) Reduce friction

(d) Reduce heat generation. They use rolling elements in between the moving and the

stationary elements of the machine.

Advantages when compared with friction guides are

(a) Low frictional resistance

(b) No stick-slip

(c) Ease of assembly

(e) High load carrying capacity

(g) High traverse speeds

The main disadvantage of these guideways as compared tofriction guideways is their lower damping capacity.


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Recirculating Ball Bushings:

o Various kinds of ball bushes are available like closed type and

open type etc.o They are available with or without seals.

Linear Bearings with Balls and Rollers:

A number of machines use rollers to provide for a rolling motionrather than a sliding motion.

The rollers are in contact with the guideway machined on acasting of the machine.


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These are very effective in providing smooth and easy movement,but still require an accurate form to be machined on castings.

The surfaces in contact with the rollers have to be hardened andshould have a smooth texture.

Linear bearings with balls:

Selection of Guideways:

The selection of guideways for a particular applicationbasically depends upon the requirements of the load

carrying capacity, damping and the traverse speed.

In order to get the maximum benefit, most of the machinetool manufacturers make use of a combination ofantifriction and friction guideways with turcite/PTFE lining.


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Such a combination improves the load carrying capacitythrough the use of antifriction guideways and the dampingproperty through the use of friction guideways.

Other types of Guideways:

(a) Hydrostatic guideways

(b) Aerostatic guideways

Hydrostatic Guideways:

In hydrostatic guideways, the surface of the slide isseparated from the guideway by a very thin film of fluidsupplied at pressures as high as 300 bar.

Frictional wear and stick slip are entirely eliminated.

A high degree of dynamic stiffness and damping is

obtained with these guideways. Their application is limited due to high cost and difficulty

in assembly.

Aerostatic Guideways:

In aerostatic guideways, the slide is raised on a cushion of

compressed air which entirely separates the slide and theguideway surfaces.

The major limitation of these type of guideways is theirlow stiffness, which limits their use to positioning


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applications only, e.g. a Coordinate Measuring Machine(CMM).

FEED DRIVES:

On a CNC machine the function of feed drive is to providemotion to the slide as per the motion commands.

The feed drive consists of:

(a) Servomotor

(b) Mechanical transmission system

Servomotor:

DC Servomotor AC ServomotorDC servomotors and driveswere used most commonlyon all CNC machines.

AC servomotors provide aconstant torque over theirentire speed range, requireless maintenance due tobrushless operation.

These servomotors provideexcellent speed regulation,high torque and efficiency.

Provides better responseand dynamic stiffness, and ahigher reliability comparedto DC servomotors


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Mechanical Transmission System:

The mechanical transmission system of a feed drivecomprises all the components which are in the force andmotion transmission paths from drive motor to the slide.They are:

o Elements to convert the rotary motion to a linearmotion (recirculating ball screw nut or rack-and-pinion system)

o Torque transmitting elements (gear box or timing beltand - couplings)

The main criterion to be considered in the design of amechanical transmission system is to keep the transmission

errors to a minimum. The essential requirements for this are:o High natural frequencyo High stiffnesso Sufficient dampingo Low frictiono Backlash free operation


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Elements Used to Convert the Rotary Motion to a LinearMotion:

Screw and nut: The screw and nut system is effective formedium traverses. With longer traverses the screw sagsunder its own weight.

Conventional vee, trapezoidal or square thread forms arenot suitable for CNC machines, because the sliding actionof the contacting surfaces of the thread on the screw and

nut results in rapid wear and the friction is high. Theefficiency of these screws is only of the order of 40%.

There are two types of screw and nut systems used on theCNC machine tools, They are recirculating ballscrews androller screws

Two types of thread forms:


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Recirculating Ballscrews:

The recirculating ballscrews are widely used on CNC machine

tools because of the following advantages: Low frictional resistance

Low drive power requirement

Little temperature rise

Less wear and hence longer life

No stick -slip effect

High traverse speed

High efficiency

The balls rotate between the screw and the nut and atsome point are returned to the start of the thread in thenut.

Two types of arrangements for returning the balls, i.e. the

recirculating arrangements are shown in Figs 5.26 and 5.27.

In the arrangement shown in Fig. 5.26 the balls arereturned through an external tube, whereas, in thearrangement shown in Fig. 5.27 the balls are returned tothe start through a channel inside the nut.


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Two types of recirculating arrangements

Fig. Recirculating Ballscrew Zero backlash with ballscrews is achieved by fitting two

nuts as shown in Fig. 5.28.


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The nuts are forced apart, or alternatively squeezedtogether, so that the balls in one nut contact one side ofthe threads in the nut and the balls in the other nut

contact the opposite side of the threads.

The efficiency of a recirculating ballscrew is of the order of90% and is obtained by the balls providing a rolling motionbetween the screw and the nut.

friction guideways

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