Int. J. Engg. Res. & Sci. & Tech. 2015

Amit U Pawar et al., 2015 ISSN 2319-5991 www.ijerst.com Vol. 4, No. 2, May 2015 © 2015 IJERST. All Rights Reserved

Research Paper

DESIGN OF LINEAR MOTION GUIDEWAYS Amit U Pawar1*, Apurav A Wagh1 and D U Patil1

*Corresponding Author: Amit U Pawar  [email protected]

The Guideway is one of the important elements of machine tool. The main function of the guideway is to make sure that the cutting tool or machine tool operative element moves along predetermined path.The Linear Motion Guideways provide a smooth and linear motion in machine tools, due to which higher accuracy and precision can be obtained. The paper deals with the study of Linear Motion Guideways, its structure, calculations of Guideways, Advantages and Applications in various machine tools where precision is of great importance. Keywords: Linear Motion Guideways, Construction, Advantages, Calculations, Selection of

Guideways, Mounting of LM Guideways, Lubrication, Applications

INTRODUCTION

Sliding Contact Linear Motion Bearings

Linear rolling ball guideway is a key component of many machines. A linear guideways allows a type of linear motion that utilizes rolling elements such as balls or rollers.By using recirculating rolling elements between the and the block, a linear guideway can achieve high precision linear motion .Compared to a traditional slide, the coefficient of friction for linear guideway is only 1/ 50. Because of the restraint effect between the rails and the blocks, linear guideways can take up loads in both the up/down and the left/right directions. With this features, linear guideways can greatly enhance moving accuracy, especially, when accompanied with precise ball screws.

This type is the oldest, simplest, least expensive way, and it still has wide range of applications. In general all sliding contact bearings have greater friction coefficient than other types described hereunder, and because of this, they are considered to be inferior to these guides for precise positioning applications. With lubricant forcibly maintained between two relatively moving objects, a relative speed helps to pull in lubricant, and thereby, a thin film is formed.

Rolling Element Linear Motion Bearings This type decreases friction utilizing rolling contact via rolling elements (balls, rollers, etc.) that are placed between two relatively moving objects. There are many specialized manufacturers and each manufacturer provides wide variety of

DIFFERENT TYPES OF LINEAR MOTION BEARING 1

Department of Mechanical Engineering, Universal College Of Engineering and Research, Pune.

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products. Because of its superiority over the sliding contact linear motion bearings as described hereunder and because of its availability, this type has won the position as an essential component for the equipment that requires highly accurate positioning operation.

moving. 2. Long life with highly accurate motion: With a traditional slide, errors in accuracy are caused by the counter flow of the oil flim. Insufficient lubrication causes wear between the contact surfaces, which become increasingly inaccurate. In contrast, rolling contact has little wear; therefore, machines can achieve a long life with highly accurate motion.

Hydrostatic or Aerostatic Linear Motion Bearings When extremely accurate and quiet operation must be attained, a guide without mechanical contact between its elements is often chosen. With pressurized fluid forcibly supplied between two relatively moving objects, one of them is kept floating by the fluid. Depending upon the fluid in use, it is classified in aerostatic and hydrostatic linear motion bearings. Although this type of guides is very advantageous for particular purposes, it is usually costly, difficult in manufacturing, and requires expensive auxiliary apparatus. Yet, this type is sometimes used for ultra-precision machines.

3. High speed motion is possible with a low driving force: Because linear guideways have little friction resistance, only a small driving force is needed to move a load .This result in greater power saving , especially in the moving parts of the system .this is especially true for the reciprocating parts. 4.Equal loading capacity in all directions: With this special design, these linear guideways can take loads in either the vertical or horizontal directions.Conventional linear slides can take small loads in the parallel to the contact surface.They are also more likely to become inaccurate when they are subjected to these loads.

Magnetic Linear Motion Bearings By means of magnetic force, either repelling or attracting, one of the two relatively moving objects is kept afloat. Use of electromagnet makes it costly and ineffective in energy consumption. This type has very limited applications.

5. Easy installation: Installing a linear guideways is fairly easy. Grinding or milling the machine surface, following installation procedure, and tightening to their specified torque can achieve highly accurate linear motion.

ADVANTAGES OF LINEAR GUIDEWAYS

6. Easy lubrication: With a traditional sliding system, insufficient lubrication causes wear on the contact surface. Also, it can be quite difficult to supply sufficient lubrication to the contact surfaces because finding an appropriate lubrication point is not very easy with linear motion guideway, grease can be easily supplied through the grease nipple on

1. High positional accuracy: When a loaded plate is driven by a linear motion guideway, the frictional contact between the loaded plate and the bed is rolling contact. The coefficient of friction is only 1/50th of traditional contact, and the difference between the dynamic and the static coefficient of friction is small. Therefore, there would be no slippage while the table is

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the linear guideway block. It is also possible to utilize a centralized oil lubrication system by piping the lubrication oil to the piping joint.

conductor equipment, medical equipment. · RG series-CNC machining centres,heavy duty cutting machines, CNC grinding machines, injection moulding machines, electric discharge machines, wire cutting machines, plano millers.

7. Interchangeability: Compared with traditional boxways or v-groove slides,linear guideways can be easily replaced should any damage occur. For high precision grades consider ordering a matched,non –interchangeable, assembly of a block and rall.

• Selection of accuracy – Classes: C, H, P, SP, UP depends on the accuracy of equipment.

HOW TO SELECT LINEAR GUIDEWAY

• Determines the size and the number of blocks – Dynamic load condition

Identify the Condition

·

Type of equipment

· Magnitude and direction of loads

·

Space limitation

· Moving speed ,acceleration

·

Ac curacy

· Duty cycle

·

Stiffness

· Service life

·

Travel length

· Environment

– If accompanied with a ballscrew, the size should be similar to the diameter of ballscrew. For example ,if the diameter of the ballscrew is 35 mm. then the model size of linear guideway should be HG35. • Calculate the max. load of block – Make reference to load calculation examples, and calculate the max.load.

Selection of Series

– Be sure that the static safety factor of selected guideway n is larger than the rated static safety factor.

· HG series-Grinding, milling and drilling machine, lathe, machine center. · EG series–Automatic equipment, high speed transfer device, semiconductor equipment, wood cutting machine, precision measure equipment.

• Choosing preload – Depends on stiffness requirement and accuracy of mounting surface.

· QE/QH series-precision measure equipment, semiconductor equipment, Automatic equipment, laser marking machine, can be widely applied in high-tech industry required high speed, low noise, low dust generation.

• Identify stiffness

· WE series-Automatic device, transportation device, precision measure equipment, semiconductor equipment, blow moulding machine, single axis robot-robotics.

• Calculating service life

– Calculate the deformation () by using the table of stiffness values, choosing heavier preload and larger size linear guideways to enhance the stiffness.

– Calculate the life time requirement by using the moving speed and frequency .

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ball slide and, further, go through the circulation circuit to the tip of the end cap of the other side and then, return to the ball grooves of rail and ball slide. Thus, the balls repeat their endless circulation motion.

– Make reference to the life calculation example • Selection of lubrication – Grease supplied by grease nipple – Oil supplied by piping joint

LUBRICATION

• Completion of selection

When using an LM system, it is necessary to provide effective lubrication. Without lubrication, the rolling elements or the raceway may be worn faster and the service life may be shortened.

STRUCTURE OF LINEAR MOTION GUIDEWAY Figure 1: Structure of LM Guideway

A lubricant has effects such as the following: 1. Minimizes friction in moving elements to prevent seizure and reduce wear. 2. Forms an oil film on the raceway to decrease stress acting on the surface and extend rolling fatigue life. 3. Covers the metal surface to prevent rust formation.

The linear guides consist of a mechanism in which steel balls are circulated infinitely to enable an infinite stroke of ball slides theoretically. Balls roll along the ball groove formed on a rail and a ball slide and there, they are scooped at the point A by the tip of an end cap. There, they are forced to change their circulating direction by a return guide of the end cap and guided to a circulating hole provided inside of the ball slide. The balls continue to pass through the hole to the other end of the

To fully bring out an LM system’s functions, it is necessary to provide lubrication according to the conditions. Even with an LM system with seals, the internal lubricant gradually seeps out during operation. Therefore, the system needs to be lubricated at an appropriate interval according to the conditions.

CALCULATION OF GUIDEWAYS WITH ONE EXAMPLE

Figure 2: Linear motion Guideways with Two Block

• Nomenclature d = diameter of ball (mm) K = load coefficient (kgf/ cm2) P = load capacity (N) Pmax = maximum pressure on ball This article can be downloaded from http://www.ijerst.com/currentissue.php 268

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 = deformation

• Pressure on guideways

N = normal load (Kg)

Pmax = 91.8

3

P /( K 2  d 2 )

= 91.8

3

9.6 /(0.62  42 ) 

r = radius of ball (mm) Figure 3: Rail (LM guideways)

= 91.8 × 1.18 = 108.84 N/mm2 For 48 balls Figure 4: Rail Block (LM Guideways)

P max = 108.84× 48 = 5224.32 N/mm2 This load acts on one block. • To calculate standard deformation The relationship between deformation and normal load for steel guides with ball slideways;  = 1.48

• Design of guideways

N 2 / r

N = 20 kg ;( normal load) as per graph

Series: MR W 25 A Diameter of ball = 3.94

r = 2 mm (radius of ball)

pl chk figure 1,  = 1.48 3 20 2 / r 2, 3, 4, 7, 8 are  = 8.65 × 10–3 mm not Here, the standard deformation from catalogue mentioned for radius 2 mm is 9.76 × 10–3 mm and calculated in the deformation is 8.65 × 10–3 mm. text

mm

Length of the block = 60 mm Number of balls = 12 balls Total number of ball in contact = 12×4 = 48 Assume K= 6 kgf /cm2

Therefore, the design is safe.

= 0.6 N /mm2 Load on each ball = K.d 2 = 0.6 × 4

3

MOUNTING OF LM GUIDEWAY

2

Linear guideways have equal rating in radial, reverse radial and lateral direction. The application depends on Machine requirement and load direction. In typical methods of mounting always rails are stationary and blocks are movable as

= 9.6 N Load carrying capacity of 48 balls P = 9.6 × 48 = 460.8 N

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Figure 5: Structure of LM Guideway

3) It required less mounting area.

APPLICATIONS OF LM GUIDEWAYS Following are few Applications of LM Guideways: 1. CNC Lathe. 2. The application of the linear guideways is very extensive, such as automation equipment, heavy-duty carry equipment, heavy-cut machining tool, CNC grinding machine,

shown in Figure 5. But there are some limitations of above mounting like we get limited stroke for guideway,

3. Large-scale planning machine and machining center with the demand of high rigidity and heavy load.

due to continues movement of block load on it is always fluctuating it may leads to failure of guideway, etc.

4. It can also be used in Aluminium Extrusion Press.

By understanding above reasons we are replacing nominal method of mounting with some

5. Industrial Robots

modified aspects which are shown in Figure 6. In

6. Semiconductor Machines- PCB Driller

this modified method of mounting the blocks are

7. Other Machines-Measuring Machine, Transporting Machine, Welding Machine, Medical equipments, Test equipments, etc.

going to be fixed and rails are movable on block. This lead to overcome all limitations of Figure 6: Structure of LM Guideway

Figure 7: Vertical Milling Machine Center

conventional method .

Figure 8: Base of VMC Showing LM Guideways

Advantages of Modified Method of Mounting are: 1) Stroke length of guideways increases up to 130%. 2) Loads on Blocks are stationary instead of Fluctuating, which increases life of the guidway. This article can be downloaded from http://www.ijerst.com/currentissue.php 270

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8. Injection Moulding Machine.

Guideway (LM Guuideway) that consists of Guide Rail, Guide Block, Balls, End cap, End seal. LM

9. Used in CNC machining Centre.

Guideways are used on various machine tools

Following are few of the Manufacturers of LM Guideways:

such as CNC Machining Centre, CNC Lathe, Industrial robots, Semiconductor machines, other

NSK Ltd.

machines like Measuring machine, Transporting

HIWIN Linear Motion Products & Technology

machine, welding machine, etc., and Selection

ABBA LINEAR TECH Co. Ltd.

procedure of LM Guideways, Calculations of LM Guideways also few Manufacturers of LM

According to Manufacturers Catalogue the LM Guideway can be selected by following their selection procedure as mentioned in the particular catalogue.

Guideways are mentioned.

REFERENCES 1.

HIW IN Linear Motion Products and Technology.

2.

NSK Linear Guide Tutorial.

3.

NSK.CO.Ltd.

4.

Mehta N K, Machine Tool Design And Numerical Control, 2nd Edition, Tata McGrawHill Publishing Co. Ltd., New Delhi.

5.

ABBA Linear Co. Ltd.

CONCLUSION The paper discusses about the Linear Motion Guideway, a high accuracy and precision motion is obtained. It has various advantages like Easy Lubrication, installation, interchangeability, long life with high motion accuracy, high positional accuracy, etc. The structure of Linear Motion

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