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What is a Linear Guidance System

Linear guidance systems are construction elements that are used to enable parts of machines to perform a translation according to a fixed track.

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Purpose of a guide

• The guiding of a component • The supporting of a component

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Different types

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Different Linear Guidance Systems

• Linear ball bearings

• Profile rail guides

• Precision rail guides

• Speedi-Roll

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Selection criteria

• Load capacity• Stiffness• Speed and acceleration• Guiding precision

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Form of contact• Ball on shaft

Small contact area

Low load capacity

Example: Linear Ball Bearing LBC•Ball within a groove

Larger contact area

High load capacity

Example:Profile Rail Guides LLB•Roller on flat surfaceLargest contact area

Maximum load capacity

Example:Precision Rail Guides LWR

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Load capacity versus precision

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Load capacity versus deformation (system stiffness)

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Velocity

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Various guiding systems according to the requirements of speed and accuracy of the application

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Applications 1

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Applications 2

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Applications 3

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Applications 4

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FLAT GUIDEFlat guides allow a displacement to be made along a fixed track. The stationary guide has a rectangular shape (cross-section) and is called a ruler. A slide is placed on top of this ruler, along which it can slide.

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Advantages & Disadvantages

A flat guide: - Is suitable for greater lengths. - Can take very high pressure forces. - Is easy to produce 。

A flat guide: - Needs adjusting strips or adjusting wedges in order to set the clearance. - Does not readjust the clearance automatically. - Acts with much friction.

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Setting the clearanceThe setting of the clearance can be carried out in different ways: - With adjusting strips. - With adjusting wedges.

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Adjusting stripsThe most simple solution is an adjusting strip. The adjusting strip is a small rectangular strip, sometimes made of bronze, often of plastic. The adjusting strip can be adjusted in two ways, namely: - With draw bolts and pressure bolts.- With adjusting screws.

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Draw bolts and pressure bolts

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Adjusting screws

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Adjusting wedges

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Measuring tools

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Lubrication of the flat guide

•To provide sufficient lubrication between the slide and the ruler, oil is generally used. Greases are often too viscous (very low viscosity) and so can hardly penetrate this gap.

Today there are also systems available that operate ‘dry’ (without lubrication). These employ special materials, such as PA (nylon) and PTFE (Teflon), which have a lubricating property. However, these systems can only operate without lubrication at low loads and low speeds. With high loads and speeds, lubrication is still required.

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Linear Ball BearingsLinear Ball Bearings is a ball guide bush that is a bearing bush for shafts for rectilinear guiding. The movement is made possible because rolling elements (recirculating balls) are located on the inside of the bush. These balls roll between the shaft and the bush. Hence a roller bearing

and a linear movement of machine parts are possible.

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PurposeThe purpose is to provide a bearing for a machine part called upon to make a translating movement. The straightness of the displacement depends on the shaft, along which the ball guide-bush rolls. The straighter the shaft, the better the rectilinear movement. For accurate displacements, a ground shaft is required.

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Advantages & Disadvantages

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The advantages of a ball guide bush are: - Easy to mount on the shaft. - A long operational life. - Can be heavily loaded. - Creates little friction.

A disadvantage of a ball guide bush is:For use on a slide, it has to be combined with a second ball guide bush or with another guide, because it does not prevent rotation.

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Type of Linear Ball Bearing

• Closed linear ball bearing

• Open Linear ball bearing

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Open and closed linear ball bearings

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Mounting of linear ball bearings

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Mounting the ball guide bush in the bearing housing

1. The pressing-in of the ball guide bush is done in the same way as with other rolling bearings.

2. Ensure that the hole of the bearing housing, the

mounting mandrel and the outside of the ball guide bush are clean. This is very important!

3. Place the ball guide bush on the mounting mandrel

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4.Place the ball guide bush (with mounting mandrel) above the hole of the housing and push it carefully with the mandrel into the hole. Ensure whilst doing so that the ball guide bush is vertically aligned with the hole.

5.With a plastic hammer, softly tap the ball guide bush into the housing. The pressing-in can also be done with a small hand-operated press or with a ball guide bush puller.

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Placing the ball guide bush over the shaft

1. Ensure that the shaft is clean and free from burrs.

2. Slide the ball guide bush carefully over the shaft. 。

3. Prevent the ball guide bush from sliding off the shaft.

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Profile rail guides

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Profile rail guides

Profile rail guides are ball-recirculation guides for high loads and high stiffness.

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Types of Profile Rail Guides

• Profile rail guides with ball bearings

• Profile rail guides with cylindrical bearings

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The advantages and disadvantages of profile rail guides with ball bearings with respect to one with cylindrical bearings

Advantages:• operates with less friction• operates with a higher maximum speed

Disadvantages:• is less accurate• cannot take such heavy loads

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Advantages ： - A long operational life.- Small overall dimensions.- Is absolutely straight (no deviations). - Slides operate with very little friction. - Slides can move at high speed. - Slides can take heavy loads (particularly slides with cylindrical bearings). - They can be produced in large lengths by coupling the rail guides. - They can be placed in different positions (horizontally,

vertically, diagonally, etc).

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Disadvantages - Expensive. - Not corrosion-resistant. - Very sensitive to inaccurate assembly. - Difficult to disassemble because the bolts, with which the guide is fastened, are covered by a protective strip or protective caps.- Does not possess an end stop to prevent the slide from running off the rail. To prevent this, a separate stop must be designed.

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Coupling the rail• The end faces of the rail sections are ground

and numbered by the manufacturer

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Alignment of the coupled sections

• The coupled sections are easy to align. This alignment is carried out with the aid of a roller. By clamping the roller into the ground profile, the portions become accurately aligned.

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Alignment of the rails

The parallelism is not determined with the aid of a formula but is a fixed value, depending on the type and dimensions of the rail. Here the dimensions (height and width) of the rail itself are decisive. The larger the rail, the greater the tolerance. The tolerance (in parallelism) lies between 0,017 m and 0,040 m.

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Alignment of the rails

H = f x aH = max. height difference f = factor, depending on the supplier and the

type/class employed (Star: 0,0006 en 0,0008) a = distance between the two rails

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Assembling the rail guideThe rails are fastened with the aid of special mounting bolts. The bolts are delivered by the supplier and must be tightened with a prescribed tightening moment. The tightening of the mounting bolts must be done according to a certain sequence. Here the tightening of the bolts is started in the middle of the guide and is continued towards the ends. This prevents stress, and consequent distortion of the rails.

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The rails are often mounted on parallel‑ground surfaces that are provided with a centring rim. If no centring rim is present, the rails must be adjusted to ensure they are parallel. This can be done by accurately aligning one rail with respect to the other with the aid of a dial gauge.

Another method for adjusting one rail parallel to another is to use the slide. Here one rail is mounted, aligned and tightened. The other rail is then fastened finger tight and the slide placed on the rails.

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By moving the slide along the rail it can be felt at which position the slide tends to stick. A plastic hammer is then used to carefully tap the side of the rail at this position to shift the rail a little at this point.

The slide is then again moved in the same way and checked for freedom of movement. Should the slide not run freely, the wrong side of the rail has been tapped.

When the rail is properly adjusted, the mounting bolts can be tightened with a torque wrench. The tapping with the plastic hammer on the rail should be done carefully, so as to avoid damaging the rail.

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Fixing with the aid of a stopThe fastening of profile rail guides is not designed to receive lateral forces. When there is risk that this could occur, the rail and the slide must be mounted against a stop to prevent the rail from shifting. The stop has a ground stopping surface, the flatness of which is accurate. The rail and the slide are fixed (tightened) with the aid of:

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Sealing the mounting holes

After assembling and adjusting the guide, the fixing holes in which the bolts are countersunk are sealed. This ensures that the surface of the guide is smooth and flat. with the rail.

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StripperThe stripper works as a kind of windscreen wiper to clean away dirt or liquid that falls on the rail in front of it, so that this cannot reach the rolling elements in the slide. This increases the operational life and the accuracy of the guide. The strippers create more friction, however, because of their contact with the rail.

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Different sealing methods

• Protective strip (clamped)

• Protective plugs

• Protective strip (stuck on)

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Protective stripWith protective strip, made of steel, a smooth surface is obtained. Some suppliers (among which Star) clamp this protective strip on the end face with a protective cap.

Other suppliers (among which Schneeberger) stick the protective strip to the surface of the guide.

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When the strip is clamped, a cap is mounted to prevent contamination from getting between the strip and the guide. The cap also prevents the strips from shifting. Because of the cap, these guides can no longer be coupled.

A strip that is stuck in position also prevents contamination from getting between the strip and the guide. This also prevents the strip from shifting. Removal of the strip is more difficult in the case of adhesion. An advantage, is that with this type of seal the rails can still be coupled.

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Protective plugsProtective plugs are usually employed for the sealing of mounting holes. The plugs, which are pressed into the mounting holes, are made of plastic, brass or steel.With this construction, too, it is possible to couple several sections of guides. The removal of the plugs is not easy. Their design makes it impossible to remove them undamaged.

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Plastic plugs are employed when no additional surface requirements are specified.

Brass plugs are employed when high demands are made upon the quality of the surface.

Steel plugs are employed in those applications where plastic or brass plugs cannot be used. The steel caps are made of stainless steel. 

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Disassembly of the railRail guides encounter little wear and are therefore not required to be replaced often. Because of this, disassembly of the rail does not frequently occur. The balls or cylinders in the slide wear faster than the rail. When the rolling elements are worn, the whole slide is often replaced.

The removal of the protective strip is easier than the removal of the protective plugs. Protective plugs are not designed to be removed easily. They can only be removed by first destroying them. After drilling a hole in the plug, this can be drawn from the hole with a hook. It is also possible to tap a screw thread in the hole, but this takes more time.

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Lubricating a profile rail guide

Lubrication increases the operational life of the guide. Lubrication is not only necessary to reduce the friction, but also to combat corrosion. Anti-corrosion dopes are therefore often added to the lubricant.

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DOVETAIL GUIDEA dovetail guide is a guide that is used to enable parts of machines to perform a rectilinear movement. The dovetail guide consists of two parts, namely the ruler and the slide. Both parts have the shape of an upside‑down trapezium. These parts are designed such that they fit into each other and can slide backwards and forwards. Most dovetails are designed with an angle of 50º.

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Adjusting the clearance

non-adjustable dovetail guide

adjustable dovetail guide

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Flat adjusting strip

Hook strip

Adjusting wedges

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Slide constructionsA slide construction is a construction in which a movable part, the slide, is guided by a fixed part, the guide. By this the slide can perform a rectilinear movement.

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Slide designs

• Slide with rectangular guide

• Slide with round guide

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Slides with two guides

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Avoiding over-defining

• a and b produced in one clamping

• aligning of the slide

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Function of a ball screw

Ball screws transform the rotary motion (provided to the screw by a motor) into linear motion of the nut.

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Disadvantages of a traditional screw

• Approximately 70% of the transmitted power is dissipated in heat generated by friction produced by the sliding surfaces.

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Ball screw

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Advantages & Disadvantages

Advantages:• is very accurate• can translate clearance-free• requires little maintenance

• Disadvantages:• expensive• requires special attention

by assembly

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Preload

• Without preload

• With preload

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Radial and moment loads

Any radial or moment load on the nut will overload some of the contact surfaces, thus significantly reducing life.