Machine Design #18: Linear Shaft — How to Choose the Guide Shaft for a Linear Bushing

In the previous article, I talked about the difference between an LM guide and a linear bushing.

If you use a linear bushing, you almost certainly have to think about a companion part: the linear shaft, which we can also call the linear guide shaft.

From the outside, a linear shaft is just a round shaft, polished, used for a ball bushing to run back and forth on.

But in machine design, this is not "any round steel bar will do."

The linear shaft is the surface the balls of the linear bushing roll directly on. So the material, surface hardness, roughness, diameter tolerance, support method, lubrication and working environment all directly affect the life of the mechanism.

There are faults you do not see right away. A newly assembled machine still runs. But after a few months the shaft starts to show streaks, the bushing runs with a grinding sound, the slide table develops play, or the mechanism gradually runs heavier.

This article records the points to watch when choosing and designing a linear shaft for a mechanism that uses a linear bushing.

The goal is not to memorise the catalogue, but to understand the points that can cause faults out in the field.

1. What is a linear shaft?

A linear shaft is a round shaft used as the guide path for a linear bushing or some types of linear sliding bushing.

A simple mechanism usually consists of:

2 linear shafts
4 linear bushings
2 or more shaft supports
1 slide table or moving mount

When the slide table runs back and forth, the balls inside the linear bushing roll directly on the shaft surface.

In other words, the surface of the shaft is the "running rail" of the ball bushing.

So a linear shaft has to guarantee several factors at once:

Accurate diameter
Good roundness
Good straightness
A surface smooth enough
A surface hard enough
Not rusting too quickly
Not deflecting excessively under load
Easy to lubricate and maintain

If just one of these points is off, the mechanism easily develops faults.

2. Do not use bright round steel or migaki material as a linear shaft

A fairly common mistake is to take bright round steel, cold-drawn steel, or migaki material as a substitute for a linear shaft.

To the naked eye they may look the same.

Both are round bars. Both are bright and shiny. Both have roughly the right diameter.

But in terms of guiding function they are very far apart.

A standard linear shaft is usually centerless-ground to control the diameter, roundness, straightness and surface quality. Cold-drawn steel or ordinary bright round steel, on the other hand, is not necessarily controlled tightly for the purpose of being a raceway for balls.

If you use migaki material as the shaft for a ball bushing, the following faults easily occur:

Sections where the bushing has play because the diameter is small.
Sections where the bushing runs heavily because the diameter is large.
A surface with small waviness that wears the balls and seals quickly.
Poor roundness making the motion uneven.
Ball-bushing life dropping sharply.

So if you design a mechanism using a linear bushing, do not write vaguely:

Steel shaft φ20

Write more specifically along the lines of:

Linear shaft φ20, hardened & ground, SUJ2 or equivalent

If it is a custom-manufacture drawing, you need to also specify information such as material, surface hardness, tolerance, roughness, surface treatment and length.

3. Where does centerless grinding matter?

Centerless grinding is a grinding method very often used for precision round shafts.

For a linear shaft, the purpose is not just to make the surface look shinier. More importantly it controls:

The diameter
The roundness
The cylindricity
The surface roughness
The stability as the bushing runs along the shaft

A linear bushing is a rolling-ball mechanism. The balls contact the shaft over a very small area. So if the diameter or surface of the shaft is even slightly uneven, the running feel changes immediately.

On some standard shafts, the diameter tolerance can be g6 or h5. This is a much higher precision level than ordinary round steel.

For example, on a φ20 shaft, a g6 or h5 tolerance is controlled very tightly so the ball bushing has a suitable clearance.

Machine design should not look only at the "nominal diameter." You need to understand that:

Whether a linear bushing runs well depends heavily on the real accuracy of the shaft.

4. Standard material: SUJ2

The common material for a linear shaft is SUJ2.

SUJ2 is a bearing steel with good wear resistance, suited to the contact surface for balls.

However, just writing the material as SUJ2 is not enough. For a linear shaft, the important thing is that the surface must be hardened.

The reason is that the balls in a linear bushing roll directly on the shaft. If the shaft surface is soft, the balls quickly create indentations, streaks or small flaking on the surface.

Once the shaft has a mark, the motion is no longer smooth. As the balls pass over that area many times, noise and wear gradually increase.

For a linear shaft used with a ball bushing, the surface hardness is usually around:

HRC 58 – 64

The exact figure depends on the maker and product type, but the design thinking is:

The surface must be hard enough to withstand the rolling contact of the balls.

5. Why is induction surface hardening needed?

A linear shaft is both a rolling surface and a part subject to bending.

If the entire shaft is made too hard and too brittle, it may not do well under impact or moment. So many shafts use induction surface hardening.

This method makes the outer layer of the shaft hard, while the inner core keeps a certain toughness.

Put simply:

The outer surface is hard to resist wear.
The inner core still has toughness to better withstand bending and impact.

When designing or ordering custom machining, you also need to pay attention to the concept of hardened depth.

If the hardened layer is too thin, under high load the surface layer can crack or flake off like an "eggshell." This is a very annoying kind of fault because at first the shaft still looks fine, but after some running it starts to flake or pit on the surface.

For high-load or continuously running mechanisms, do not overlook this information.

6. Machining after hardening: be careful

Another practical point is that a linear shaft, once hardened and precision-ground, should not be machined carelessly.

For example:

Milling a flat
Drilling a cross hole
Tapping
Turning a step
Cutting a groove
Local welding or heating

These operations can remove the hardened layer, cause cracking, bend the shaft, or change the existing accuracy.

If you need to machine the shaft end, an internal thread, a pin hole or a D-cut, choose a shaft type that allows the machining to be ordered pre-done from the supplier, or design from the start so that the machined part lies in the non-working region of the bushing.

Do not take a standard shaft and process it yourself by "cutting, drilling, welding to save time" if you do not understand the effect on the hardened layer and straightness.

7. Surface roughness: not just for looks

A linear shaft needs not only to be hard, but also to have a surface smooth enough.

If the surface is too rough, it is like a very fine sandpaper grinding into the balls, seals and retaining ring inside the bushing. Running it easily generates:

Noise
Increased friction
Grease that blackens quickly
Seals that wear quickly
A bushing that no longer runs lightly
Reduced life

For a shaft used with a ball bushing, the surface should usually be around:

Ra 0.4 µm or lower

It can differ by maker and product type, but this is an easy benchmark to remember when checking.

However, mirror-smooth is not always good in every case. The surface needs to be smooth enough for the balls to roll smoothly, but also needs to keep a thin film of oil/grease for lubrication.

In practice, if you buy a standard shaft from a reputable maker, the roughness is usually fine. But if you use outsourced machining, cheap goods, or goods of unknown standard, you should check this part carefully.

8. Rust prevention: SUJ2 is good but rusts easily

SUJ2 has good wear resistance, but it is not stainless steel.

In a humid environment, with water, oil mist, coolant, or an area prone to moisture condensation, the shaft rusts very easily if not protected.

Just one small rust spot on the shaft surface, and the balls passing over it are no longer smooth. The rust spot also wears the seal, dirties the grease and creates a starting point for wear.

Some common options:

Shaft type	Advantage	Note
Standard SUJ2	Good wear resistance, common, easy to buy	Rusts easily in humid environments
SUJ2 + hard chrome	Better rust and wear resistance	Watch the plating's effect on dimensions
SUJ2 + black treatment	Reduces rust, reduces light reflection	Depends on the treatment type and supplier standard
Stainless shaft / SUS440C equivalent	Suits environments with water or needing cleanliness	Load capacity and hardness may differ from standard SUJ2

No option is right for every machine.

Dry-workshop machine, normal load: standard SUJ2 may be enough. Machine near water, humidity, washdown: consider hard chrome or stainless. Machine with a camera or optical sensor: sometimes the shaft's reflection also has to be taken into account.

9. Support at both ends or support along the full length?

When using a linear shaft, there are two common support styles.

Support at both ends

This is the simplest style. The shaft is held at both ends by shaft holders or support blocks.

Advantages:

Simple structure
Low cost
Easy to install
Usable with the closed type of linear bushing
Suits short stroke, light load

The disadvantage is that the shaft works as a beam. When the load is placed in the middle, the shaft deflects.

If the stroke is long or the load is large, this deflection can make the bushing misalign angularly, generating edge loading and quick wear.

Support along the full length with a support rail

In this style, the shaft is supported along its length by a support rail underneath.

Advantages:

Greatly reduces deflection
Suits longer strokes
Stiffer than support at both ends only

Disadvantages:

Higher cost
Assembly needs attention to the mounting surface
Usually uses the open type of linear bushing
The load direction of the open type needs careful checking

Put simply:

Short stroke, light load: support at both ends may be enough. Long stroke, larger load: consider a support rail or switch to an LM guide.

10. Calculating shaft deflection: a part not to skip

With a linear shaft, many faults come not from "insufficient strength" but from excessive deflection.

The shaft has not broken. The shaft still withstands the load. But the bushing has misaligned, taken edge loading and worn quickly.

When the shaft is supported at both ends, the deflection depends very strongly on the support span length.

A basic formula for a concentrated load at the middle of a simply supported beam is:

δ = P × L³ / (48 × E × I)

Where:

δ: deflection
P: applied load
L: distance between the two support points
E: elastic modulus of the material
I: area moment of inertia of the cross-section

For a solid round shaft:

I = π × d⁴ / 64

The point to remember is:

Deflection increases with the cube of the length. Rigidity increases very strongly as the shaft diameter increases.

Put more practically, if you double the support span, the deflection can increase very strongly. Conversely, increasing the shaft diameter by one step can markedly improve rigidity.

So for a long stroke, do not choose the shaft diameter by feel. Estimate the deflection first.

A reference benchmark in ordinary transport mechanisms is to control deflection to around 0.5 mm/m or lower. For mechanisms needing more accuracy, set a stricter standard, for example 0.1 mm depending on the machine requirement.

11. Fit tolerance: shaft g6 and housing H7

This is a part very easily overlooked but important in design.

A standard linear shaft usually has a diameter tolerance such as:

Depending on the shaft type and manufacturer.

The shaft-mounting hole, holder or housing usually uses:

A common combination is:

Shaft g6 + housing H7

The purpose is to create a suitable clearance to fit the shaft into the holder/housing, then use a clamping mechanism, set screw or slit clamp to fix it firmly.

If fitted too loose:

The shaft can have play.
The shaft position is unstable.
The mechanism accuracy drops.
Running long, it can generate vibration or wear at the mounting position.

If fitted too tight:

Hard to assemble.
Can scratch the shaft.
Can deform the holder.
Hard to align two shafts parallel.
Very hard to remove during maintenance.

With a linear bushing, if you need light running, prioritise standard clearance and good alignment. If you need to reduce play, you can use an adjustable-clearance bushing or a housing with a clamping slit to create a light preload.

But preload is not just "tighten hard = good."

Over-tightening can make the bushing run heavy, hot, wear quickly, or scratch the shaft. Especially with a two-parallel-shaft mechanism, if you both clamp the bushing tightly and align the shafts poorly, the slide table very easily binds.

The design thinking should be:

Need light running: prioritise a fit that does not bind.
Need low play: use an adjustable type, but align the parallelism carefully.
Need to take large load and moment: reconsider whether to use an LM guide.
Do not use preload to cure a design that lacks rigidity.

12. Ball bushing and oilless bushing: balls are not always the choice

When using a linear shaft, you do not always have to use a ball bushing.

There are two common groups:

A ball bushing is the type with rolling balls. Friction is very low, running is light, suited to mechanisms needing speed, sensitivity and small push force.

An oilless bushing is a sliding bushing. The material is usually self-lubricating metal, engineering plastic, or an alloy with a solid lubricant. Friction is higher than a ball bushing, but in many cases it handles static load, impact and dirty environments better.

A quick comparison:

Criterion	Ball bushing	Oilless bushing
Mechanism	Rolling balls	Sliding surface contact
Friction	Very low	Higher
Speed	Good	Depends on material
Impact resistance	Weaker	Better
Dusty environment	Can jam if debris enters the balls	Usually handles dirt better
Shaft requirement	Needs a hard, smooth shaft	Still should use a good shaft, but depends on the bushing material
Application	Light, fast mechanism, needing small push force	Slow mechanism, large load, dust, impact

If the mechanism runs in an environment with a lot of dust, debris, welding, powder or strong impact, a ball bushing is not necessarily the best choice.

Sometimes an oilless bushing is more durable.

13. Life and the 50 km / 100 km rating

When comparing linear-bushing life, pay attention to the basic dynamic load rating / basic rated life in the catalogue.

An easy point to confuse is that not every maker uses the same rated-distance standard.

Some documents use the 50 km standard. Some documents use the 100 km standard.

If you take the specifications of two different makers and compare them directly without checking the rating standard, you very easily misunderstand.

When calculating life, you need to look at:

Basic dynamic load rating C
The actual load applied to the bushing
The load factor from impact/vibration
The load direction
The shaft hardness
The lubrication condition
The dust, water, heat environment
The 50 km or 100 km life standard

In real machine design, do not put absolute faith in the calculated life figure if the running conditions have impact, dust, poor lubrication or poor alignment.

The catalogue is the starting point. The field is where the design is really tested.

14. Fretting: micro-motion wear when the stroke is too short

A fairly annoying fault on a linear shaft is fretting, which can be understood as micro-motion wear.

This phenomenon usually happens when the mechanism only oscillates back and forth with a very short stroke, or when the machine is stopped but has small repeated vibration.

Because the stroke is too short, the balls do not roll over enough distance to distribute grease evenly. The contact region is repeated continuously on a few very narrow lines on the shaft. Over time, brown, black or streak marks appear along the ball's running line.

Fretting can cause:

The shaft to wear along a streak
The bushing to make noise
The grease to blacken quickly
The mechanism to no longer run smoothly
Reduced life even though the load is not large

A maintenance trick in some cases is to rotate the shaft position.

Because the balls usually only run on a few fixed contact lines around the shaft circumference. If the structure allows removing and rotating the shaft by an angle, for example 45 degrees, the balls will run on a different, less worn surface region.

Of course this is a maintenance measure to extend life, not a substitute for correct design.

To be able to do this, from the design stage make the shaft easy to remove, rotate, reinstall and not obstructed by too many surrounding parts.

15. Lubrication: rust-preventive oil is not machine-running grease

A very common mistake is to take a new shaft and bushing and install them right away.

Many products come from the factory with rust-preventive oil. This oil layer helps storage and transport, but does not mean it is enough for long-term operation.

When assembling the machine, check the manufacturer's instructions, wipe off the preservation oil if needed, then apply the correct grease for the mechanism.

When greasing, do not just smear a thin layer of grease on the outside of the shaft for show. You need to ensure the grease gets into the ball region inside the bushing.

For a continuously running mechanism, there should be a clear lubrication cycle. For example:

By running distance in km
By number of cycles
By months of operation
By the level of dust, water, heat or load

The worse the environment, the shorter the inspection cycle must be.

Signs that lubrication needs checking:

The bushing runs with a grinding sound
The shaft has streaks
The grease blackens quickly
The slide assembly has an abnormally heavy section
The bushing is hotter than normal
The seal has a lot of dust stuck on it
There is rust or water on the shaft

Lubrication is a small thing, but it has a very large effect on the real life of the mechanism.

16. Mounting two parallel shafts: have one master shaft and one follower shaft

A linear-shaft mechanism usually uses two parallel shafts to resist rotation.

But if the two shafts are not well parallel, the slide table binds.

A common mistake is to rigidly fix both shafts per the drawing, then mount the slide table and find it runs heavy. At this point many people try to loosen the bushing, grind the hole, or add more grease. But the root cause may be that the two shafts are being forced into the wrong position.

The practical method is usually:

Choose one shaft as the master.
Fix the master shaft first.
Leave the other shaft loosely fixtured.
Mount the slide table and run it back and forth a few times.
Let the assembly find the position that binds least.
Then gradually tighten the follower shaft or follower block.

In Japanese, you may encounter a method called katagawa kijun / katagawa kari-jime (one-side reference, one-side temporary tightening), or tomo-jime, depending on how it is called in the field.

This method does not replace precision machining, but it helps reduce faults from assembly error.

For a mechanism needing high accuracy, you should still have:

A reference surface
An assembly jig
A dial gauge
A bolt-tightening sequence
A standard for checking the sliding force after assembly

17. Vertical axis: you must think about free fall

If a linear shaft is used for a vertical axis, you must not think only about guiding.

You need to think about the situation of a power loss, air loss, drive breakage, loss of control or a motor-brake fault.

For a vertical axis, consider:

A motor with a brake
A shaft-clamping mechanism
A cylinder with a lock
A counterbalance
A mechanical stroke limit
A shock absorber
A sensor to check the position
An anti-drop mechanism for maintenance

A linear shaft and bushing are not a safe load-holding mechanism. They are only a guiding mechanism.

If the load can fall and damage the machine or endanger people, you need a separate anti-drop mechanism.

This is a safety design part, and should not be left until the trial-run stage to think about.

18. Choosing a maker and a purchasing strategy

A linear shaft is a standard part, but each maker has different strengths.

For example:

Maker	Common strength	Suits
MISUMI	Easy to select, many machining options, fast delivery	Jigs, test machines, designs needing schedule
THK	Good technical documentation, high reliability	Important machines needing stability
NB	Strong in shafts and slide bushings	Round-shaft mechanisms, replacing old equipment
IKO	Many compact, precise products	Small mechanisms, equipment needing to save space

In machine design, choosing the "top-tier" maker is not always optimal.

You need to balance:

Price
Reliability
Delivery time
The ability to re-purchase later
Whether there are machining options
Whether there is a replacement available in the export country
Whether there is clear technical documentation

If the machine is exported or installed abroad, choose a type that is easy to re-purchase locally or has a clear equivalent part number. A cheap component that later cannot be bought for replacement is also a risk.

19. A checklist before finalising the design

This section should be used as a checklist before releasing the drawing or ordering components.

19.1. Check the shaft

Is the shaft a hardened & ground linear shaft?
Are you mistakenly using bright round steel or migaki material?
Is the material suitable: SUJ2, hard chrome, stainless or a special treatment?
Is the surface hardness enough for a ball bushing?
Do you need to check the hardened depth?
Is the surface roughness suitable?
Is there rust prevention if the environment is humid, wet, oil-misty or has washdown?
If there is a camera/optical sensor, does the shaft surface cause troublesome reflection?

19.2. Check the support structure

Is the shaft supported at both ends or along the full length?
If supported at both ends, has the deflection been calculated?
Does a long stroke need a support rail?
Is the shaft diameter rigid enough, or just chosen by feel?
Is the distance between the two supports too far?
Is the load placed near the middle of the span?
Is the workpiece off-centre, causing moment?

19.3. Check tolerance and assembly

What tolerance does the shaft use: g6, h5 or another?
Does the housing/holder suit H7 or an equivalent tolerance?
Is a reasonable shaft-clamping mechanism designed?
Is a too-loose fit that causes play avoided?
Is a too-tight fit that makes alignment and maintenance hard avoided?
If reducing play is needed, is an adjustable-clearance bushing used correctly?
Are you using preload to hide a lack of rigidity?

19.4. Check the two parallel shafts

Has one shaft been chosen as the master?
Does the other shaft have a mechanism allowing alignment to the master shaft?
Is there an assembly sequence: one side fixed, one side loose, trial run then gradual tighten?
After assembly, is the sliding force checked over the full stroke?
Is there any point that runs abnormally heavy?
Is there enough space to insert a dial gauge or inspection tool?

19.5. Check the bushing and environment

Ball bushing or oilless bushing?
If the environment has dust, debris, welding, powder or impact, is a ball bushing really suitable?
If low friction and light running are needed, does an oilless bushing increase the push force too much?
Is there a seal, cover or dust shielding for the shaft?
Is there a risk of water, oil, chemicals or moisture sticking to the shaft?

19.6. Check life and maintenance

When comparing catalogues, have you checked the 50 km or 100 km life standard?
Have you accounted for the impact/vibration factor?
Is the stroke too short, prone to fretting?
Is the grease type specified for new assembly?
Is rust-preventive oil distinguished from operating grease?
Is there a grease path or space for maintenance operations?
Can the shaft be removed, rotated or replaced without removing too many machine assemblies?

19.7. Check safety

If used for a vertical axis, is there an anti-drop mechanism?
Does the motor have a brake?
Is there a clamp, counterbalance or load-holding mechanism for a power/air loss?
Is there a mechanical stroke limit?
During maintenance, can the load free-fall?

This checklist does not need to be fully written on the drawing every time. But before finalising the design, go through it at least once. A linear shaft is a standard part, but the faults usually lie in how you use it in the real mechanism.

20. Conclusion

A linear shaft looks simple, but it decides quite a lot about the smoothness, play and life of a mechanism using a linear bushing.

The most important point is not to treat it as an ordinary round steel bar. The shaft must be accurate enough, hard enough, smooth enough and supported correctly.

For short, light mechanisms with moderate requirements, a linear shaft is a very economical and easy-to-use choice. But when the stroke is long, the load off-centre, the environment bad or the machine runs continuously, you have to calculate more carefully: deflection, fit tolerance, lubrication, rust prevention, fretting and the maintainability later on.

In short:

A linear shaft, used correctly, is cheap, simple and effective. But used with the wrong material, wrong tolerance or wrong support method, it quickly becomes a field fault.

In machine design, the parts that look simplest are the easiest to underestimate. The linear shaft is a very typical example.

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