Linear Shaft Evaluation and Design Complete Guide｜Material, Heat treatment, Surface treatment, etc.

Here, the guide mechanism is used.Linear shafts."　This is a practical know-how memo about

You may have such problems or concerns as "I want to make a guide mechanism using linear shafts, but I am not sure which material to choose," "I know the calculation formulas in the catalog, but the safety factor and life expectancy to be considered in practice are unclear," "I have experienced uneven shaft wear in the past and do not want to make the same mistake. I have experienced uneven wear of shafts in the past and do not want to make the same mistake.

In the field of machine design, the quality of equipment is determined by "structural specifications," "depth of heat treatment," or "on-site assembly know-how (greasing, etc.)" that cannot be seen from catalog specifications alone.

This article is not just an excerpt from a manufacturer's catalog, but also includes examples of troubles faced in actual design practice and "operational techniques for longer service life" not found in textbooks.

We have systematically summarized the rationale for material selection based on JIS standards, which is discussed only in fragments on other sites, the conversion logic between 50 km and 100 km ratings, and the engineering reasons why Migaki materials should not be used.　　By reading this article, we hope you will be able to select linear shafts with confidence and design them to prevent problems.

Linear shaft material and accuracy characteristics

Difference in decision between Migaki material and centerless material

One of the biggest mistakes that can be made in the early stages of mechanical design experience　The first is to use inexpensive "cold drawn steel" as a linear shaft simply because it looks similar.　　However, the manufacturing process itself is different between these two materials, and there is an unbridgeable performance gap in their functions as guide components.

in generalbrittle wood　is formed by passing the material through a hole called a die and pulling it out at room temperature.　　In most cases, this manufacturing process does not guarantee roundness (roundness) or cylindricity (straightness), with dimensional tolerances ranging only from h9 to h11.

On the other hand, products distributed as linear shafts are,Centerless grinding　This is a precision process called "g6".　　This is a method in which the workpiece is not fixed in the center hole, but is ground while being supported by the adjusting wheel and supporting blade. This enables finishing with several to dozens of times higher precision than that of ground material, such as g6 for diameter tolerance and h5 for clearance.

If a ball bushing uses a ground material as its mating shaft, the preload will be uneven due to tolerance variations, resulting in fatal problems such as rattling at certain points or, conversely, "too tight to move".　　The fine surface undulation characteristic of pultruded materials can also cause premature wear of balls and seals.　　Therefore, it is the designer's responsibility to select centerless ground shafts for sliding guides in automatic machines.

The table below summarizes a comparison of the characteristics of general milling materials and linear shafts (centerless grinding materials).

Necessity of SUJ2 and induction hardening

High-carbon chromium bearing steel (SUJ2) is the most standard material used for linear shafts in Japan.　　As the name suggests, this steel material was developed to make bearings, and has an extremely good balance of wear resistance and mechanical strength.

However, it is not enough if the material is SUJ2.To function as a linear shaft,High-frequency quenchingHeat treatment is a prerequisite for the　　This treatment is a technology that rapidly heats only the surface of the shaft using a coil and immediately afterwards quenches it to make the surface extremely hard, with a surface hardness of HRC58-64 (HV700 or higher in Vickers hardness).

Why is high-frequency hardening used instead of "Zub hardening," which hardens the entire shaft?　　The reason is that the shaft is not only a "guide" but also a "beam" that supports loads.　　If the entire shaft is hardened to the core, it will become brittle like glass when impact loads or bending moments are applied, increasing the risk of breakage.　　By hardening the surface to withstand the stress of contact with the ball, while leaving the toughness (tenacity) inside (core) in its original structure, an ideal shaft that is less likely to break and less likely to wear out can be completed.

The "effective hardened layer depth" should also be considered in the design drawings.　　This is the depth at which the hardness is maintained from the surface, usually 1mm to 2mm.　　If this layer is too thin, the surface will crack like an eggshell under high loads, causing premature delamination.

Relationship between surface roughness and guide performance

Linear shaftsSurface roughness　is an important parameter that directly affects the magnitude of sliding resistance, seal life, and even operating noise.　　Surface roughness indicates the magnitude of microscopic irregularities present on the machined surface and is usually controlled by arithmetic mean roughness (Ra).

When ball bushings are used, the recommended roughness of the shaft surface is Ra 0.2μm to 0.4μm or less.　　If the surface is rougher than this, the resistance of the ball as it rolls will increase, causing not only a rumbling noise, but also filing on the ball itself, the resin retainer, and the rubber seal.　　On the other hand, it is not necessarily true that a mirror-like surface with near-zero roughness is better.Moderate microscopic irregularities act as "oil pockets" and retain lubricantFrom.

In addition to centerless grinding, there are products that are buffed or otherwise polished to a finish to prepare the striations (grinding marks) and improve longitudinal sliding properties.　　It is important for designers to ensure that this surface roughness is properly controlled, especially when machining shafts in-house or using inexpensive shafts made overseas.

Hard chrome plating and environmental resistance

Bearing steels such as SUJ2, while having very good mechanical properties, do not contain as much chromium as stainless steels,Weakness in that it rusts easily if rust-prevention management is neglectedThere are　　In Japan's hot and humid summer, in environments prone to condensation, or inside machine tools where water-soluble coolant is splashed, shafts without surface treatment can develop red rust in a few days.

The most commonly used method to prevent such corrosion problems is "industrial hard chrome plating (flash plating).　　Usually applied with a film thickness of 5 μm to 20 μm, it provides surface hardness of HV750 or higher and excellent corrosion resistance.　　For more advanced corrosion protection or when light reflection is not desired, such as in optical equipment, "low-temperature black chrome plating (Raydent treatment, etc.)" is selected.　　This is an ultra-thin film of 1μm to 2μm, yet it has strong rust-preventive power, and has the advantage of minimizing the effect of film thickness on tolerances.

For food machinery and lines that require cleaning, there is also the option of using stainless steel (SUS440C equivalent) as the base material itself.　　SUS440C can be hardened to about HRC56 by quenching, but its load rating tends to be lower than that of SUJ2. Select the appropriate surface treatment and material by determining the balance between the severity of the operating environment and the required load capacity.

Fixing method and design of linear shafts

Fixing method for linear shafts

From a structural mechanics standpoint, there are two main types of methods for securing the linear shaft to the equipment frame: "floating support at both ends" and "continuous support (rail support).　　Which one is adopted is determined by the conveying distance (stroke) and the required level of rigidity.

In the double-ended support method, only the ends of the shaft are secured with shaft holders or support blocks, while the middle portion floats in the air.　　The greatest advantage of this construction is the use of cylindrical "closed type" ball bushings.　　This allows the load to be applied evenly around the entire circumference of the shaft, and costs can be kept low.　　However, since the shaft itself acts as a beam, the center of the shaft tends to deflect under its own weight and the weight of the workpiece when the shaft is long, which is a disadvantage that can cause vibration.

Continuous support, on the other hand, uses aluminum or steel support rails that support the entire length of the shaft from below.　　Since the shaft is completely fixed to the rail, deflection concerns are virtually eliminated and high rigidity can be maintained even for long conveyor lines extending several meters.　　However, it should be noted that bushings must be of the "open type" with the lower portion cut out, which reduces the load capacity in certain directions (i.e., toward the opening) and requires more effort to flatten the mounting surface.

Also,Tomotome" is an effective fixing technique for parallel installation of two shafts.　　This is done by fully securing the shaft on one side of the reference, then temporarily tightening the bolts on the other shaft (or bush block),The method of tightening the carriage after adjusting it to its natural position by moving it back and forth several times.It is.　　This prevents "kink" due to installation errors and ensures smooth movement.

Calculation method for deflection due to load

When employing a double-ended support structure, the designer should pay the most attention to the calculation of "deflection.　　Even if there is no risk of breakage in terms of strength, too much deflection will cause "one-sided contact" where the ball bushing strikes the shaft at an angle, resulting in abnormal wear and operation failure.

Assuming that the shaft is a "simply supported beam" and that a concentrated load P (N) acts on its center, the maximum deflection δ (mm) is determined by the following material mechanics formula

Here each variable is as follows

• L: Distance between fulcrums (mm)
• E: Young's modulus (modulus of longitudinal elasticity). Approximately 2.06 × 10^5 (N/mm²) for steel
• I: Secondary moment of cross section (mm⁴). I = (π × d^4) / 64 for a solid circular cross section (d is the shaft diameter)

Of particular interest in this equation is the fact that the deflection δ Proportional to span L squared Sh,Inversely proportional to the 4th power of shaft diameter d This is the point that the stiffness of the shaft is increased by a factor of 8 if the span is doubled.　　In other words, if the span is doubled, deflection increases eightfold, but conversely, rigidity can be dramatically improved by simply increasing the shaft diameter slightly.

For general conveyor systems, the allowable deflection is "0.5 mm or less per meter of span" and for applications requiring positioning accuracy, "0.1 mm or less" is the standard for design.　　If the calculation results exceed this tolerance, you should consider increasing the shaft diameter by one rank or changing to a continuous support system.

Selection of fit tolerance between shaft and hole

The "fit" between the linear shaft and the housing or bushing that holds it is an important factor that determines the accuracy and assembly of the device.　　If appropriate tolerances are not selected, problems can occur, such as positioning inaccuracy due to rattling or, conversely, damage to parts during insertion if the fit is too tight.

Typically, the outside diameter tolerance of commercially available hardened linear shafts is g6 or H5 The products are fabricated with the following two tolerances.　　g6 is the negative tolerance (e.g., -0.007 to -0.020 mm for φ20) and h5 is the zero-based negative tolerance (e.g., 0 to -0.009 mm for φ20).

In contrast, the hole diameter tolerance of the holder or block that secures the shaft is generally H7 The following is a selection of the two types of holes.　　The combination of g6 shaft and H7 hole is called "Sukimame (precision turning)The gap between the holder and the holder is about 1.5 mm (0.005 in.), which is enough to allow smooth insertion by hand.　　When fixed, the holder with a slit is generally bolted to the holder to eliminate the gap and firmly grip the holder.

In relation to ball bushings, clearance management is required according to the application.For general transport applications, the standard clearance fit is sufficient. However, if vibration or shock is anticipated or high guidance accuracy is required, a "clearance adjustment type" with a slit in the outer sleeve of the bushing can be used. This is a design method to reduce rattle to zero by adjusting the preload (negative clearance) on the housing side.　is taken.

Ball bushings and euless bushings

There are two main types of bearings selected as guides for linear shafts: ball bushings (rolling bearings) and oil-free bushings (sliding bearings).　　Since these have completely different friction mechanisms, they must be used correctly according to the operating environment and loading conditions.

The ball bushing is the rolling motion of the steel balls inside,Coefficient of friction is 0.002 to 0.005　The feature of this product is that it is extremely low, with almost no stick-slip (snagging at the start of movement).　　Therefore, it is suitable for high-speed conveyance, precise positioning, and driving with low thrust.　　However, since the ball and shaft support the load by "point contact," the surface pressure applied to one point is high,High hardness (HRC58 or higher) is essential for shaftswill be.

In contrast, oil-impregnated metal, resin, or alloys embedded with solid lubricants are used for oil-impregnated bushings, which slide in "surface contact" with the shaft.Coefficient of friction is about 0.1 to 0.2　Although higher than ball bushings, the allowable load (especially static load rating) is very high due to the large contact area, and it is effective in damping shock and vibration.　　In addition, even if foreign matter (dust, etc.) gets into the bushing, it is less likely to bite and lock like a ball,Suitable for use in adverse environments such as welding lines and foundry sites.

The table below provides a detailed comparison of their characteristics.

Reference source: Euless Industries (https://www.oiles.co.jp/products/bearing/design-support/design_selection/）

Reference source: THK Product Information (https://www.thk.com/jp/ja/products/other_linear_motion_guides/linear_bushing/）

Linear shaft life and maintenance

Lifetime calculation using rated load

Life calculation is essential to predict how long or how far the designed guide mechanism can actually be operated without trouble.　　The basis for this calculation is the "basic dynamic load rating (C).　　This is the load at the standard travel distance that the 90% can reach without flaking when a group of linear bushings of the same diameter are operated under the same conditions.

It is important to note that the definition of this reference distance varies from standard to standard. The international standard ISO 14728-1 Now let's set the reference distance to 100km but some Japanese manufacturers and older catalog data stipulate that 50km may be used as the standard.

In practice, the effective load is corrected for environmental factors such as vibration, impact, and temperature, and the hardness factor (fw) for shafts with low hardness (fH). This enables a realistic life prediction that reflects the harshness of the work site, rather than just the theoretical values in the catalog.　　Reference source: Nippon Bearing Technical Information (Japanese only)https://www.nipponbearing.com/technology/life.html）

Fretting damage and countermeasures

When linear shafts are in operation, reddish-brown or blackish streaks or wear marks may occur along the line where the ball travels. This is most likely not just wear, but a specific type of damage called "fretting" (micro-motion wear).

Fretting is caused by repetitive micro reciprocating movements (short strokes) and micro vibrations while the equipment is stopped.　If the motion is too small, a new lubricating film is not formed between the rolling elements and the raceway, and the oil film runs out.　　In this state, direct metal-to-metal contact generates microscopic wear particles, which oxidize and act like abrasives, causing rapid wear.

An effective field technique for this problem is "shaft phasing (rotation).Due to the construction of the ball bushing, there is a fixed line of strips (lines) that the ball contacts.　　In other words, of the entire circumference of the shaft, only a few lines are actually worn.　　The ball's running path can be changed to a new (unused) surface by loosening the shaft's fixing bolt and turning the shaft 45 degrees in the direction of rotation and re-fixing it during periodic maintenance.　However,Since this is designed to be handled on site, the design must have a structure that allows easy insertion and removal (and replacement) of the linear shaft.

This can substantially extend service life without incurring parts replacement costs.　　Another effective measure is to select a wear-resistant grease that is resistant to fretting.

Proper greasing procedure

Are you thinking, "I bought new linear shafts and bushings, so I can just assemble them as is? This is a trap that beginners easily fall into.　In many cases, the oil applied at the time of shipment is an "anti-rust oil" and its performance as a "lubricant (grease)" for sliding is insufficient.

The correct procedure is to first wipe the rust-preventive grease clean with a rag, etc., and then seal the appropriate grease.　　For general industrial machinery, "lithium-based soap base grease (e.g. Showa Shell Albania Grease S2, etc.)" is recommended as standard.　　When applying the grease, it is ideal to not only apply it to the surface of the shaft, but also to apply the grease directly to the ball row (cage gap) inside the bushing to spread it over the entire ball.

The frequency of maintenance depends on the operating environment and operating rate, but as a general guideline, lubrication "every 1,000 km" or "every 3 to 6 months" is recommended.　　However, when used on a vertical axis or in an environment subjected to dust or water droplets, the grease deteriorates and runs out more quickly, so the inspection cycle should be set shorter.　　Greasing not only prevents wear, but also has a sealing effect to prevent intrusion of foreign substances and rust, making it the most cost-effective maintenance activity for stable equipment operation.　　This is an external blog, but the Machine Assembly Room is a good place to start.Grease Lubrication Method　He wrote an article about the

Characteristics of MISUMI and other domestic manufacturers

There are many world-class linear shaft and linear motion component manufacturers in Japan.　　A wise designer must understand the characteristics of each company's areas of expertise and services and use them according to the purpose of the design (whether cost or performance is the priority).

MISUMI is the "king of standardization" in the factory automation (FA) industry.　　You can instantly specify lengths in 1mm increments and additional processes such as threading, keyway and flat machining on the web catalog, and the model number, price and delivery date are instantly determined. This overwhelming speed, which enables next-day shipment for in-stock items, is a tremendous advantage in the design of prototypes, sudden design changes, and custom-made jigs.

THK is the world's first pioneer in the commercialization of linear motion guides (LM guides), and its technological strength and reliability are among the best in the industry.　　We have an extensive lineup of linear bushings, and in particular, our technical data is outstanding.　　In the design of mass-production machines that are subject to high loads and important equipment that absolutely must not malfunction, we are able to provide the best solutions,High reliability can be assured by selecting THK products.

Nippon Bearing (NB) is a specialist in round shaft guides, known for its "Slide Bush" brand.　　The company's unique "TOPBALLThe "M" series has a significantly improved load rating compared to conventional products by devising the contact structure with the ball.　　The manufacturer also offers a wide variety of inch-size and special environment-compatible products to meet niche requirements.

The table below summarizes the features of the major manufacturers.

Linear Shaft Design Summary

As we have explained, the design of a guide mechanism using a linear shaft is profound in that it appears to be a simple combination of a bar and a bearing.　　Finally, we summarize the important points explained in this article.　　Please use these as a checklist to ensure a reliable design.

• Do not use grinding materials for cost reduction, but always select linear shafts that are hardened and ground.
• The basic material is standard SUJ2, but hard chrome plating or SUS440C should be considered for wet environments.
• The surface hardening layer of the shaft (high-frequency quenchingUnderstand the importance of the annealing process (see Figure 1), and consider annealing, etc., when post-processing.
• Deflection calculations must be performed because "deflection" is more dominant than "strength" in double end-supported designs.
• For long strokes where deflection exceeds the permissible value (e.g., 0.5 mm/m), use continuous support (rail support)
• Fit tolerances are based on shaft g6 and housing H7 to ensure proper clearance.
• Use ball bushings for high speed and precision, and euless bushings for heavy loads and adverse environments.
• In life expectancy calculations, note the difference in rated load criteria (50 km vs. 100 km) between manufacturers, and convert and compare.
• Periodic shaft phase rotation (rotation) as a measure against fine wear (fretting)
• Thorough initial rust-preventive oil removal and appropriate greasing (lithium type recommended) to prevent oil film loss
• When using 2 axes, "double-tightening" with one side standard and one side temporary tightening is performed to ensure smooth movement.
• When used on a vertical axis, be sure to provide a brake mechanism or clamp to prevent falling.
• Shaft surface roughness is maintained below Ra0.4 to reduce aggression to seals and balls
• Understand manufacturers' strengths and develop procurement strategies, such as MISUMI's short delivery times and THK's high reliability
• All of these factors are considered in a balanced manner, and the design must meet the required reliability without excessive quality. Aiming for

That's it.