Linear shafts: Selecting the right material, hardening, and surface treatment

Linear shafts perform demanding tasks in industrial applications: They permit precise and repeatable linear movements under high mechanical loads. To meet these requirements, proper material selection, hardening, and surface treatment is critical. All of these factors directly affect the service life, precision and performance of a linear shaft. This article introduces these three aspects and highlights their dependencies so you can select the best linear shaft for your application.

Linear shafts in detail

Linear shafts in combination with linear bearings (e.g. plain bearing bushings and linear ball bearings) act as the linear guide for axial movements. They are usually made of precision steel, but other materials are also conceivable. There are several ways to integrate linear shafts into a system. Shaft holders are just one of the many options, depending on the shaft form. For more information, see our article on linear shafts: Shaft ends and linear shaft mounting options.

Hollow shafts (pipe shape) represent a special shape for linear shafts. Hollow shafts have a hollow interior over their entire length. This uses less material, thus reducing the weight of the linear shaft. Linear shafts act as guide shafts for linear bearings. When used properly, they permit highly accurate linear motion guidance for applications with high demands for smoothness and precision. In order to meet this requirement and to ensure reliable guidance of the linear bearing over its entire service life, the interaction between material selection, hardening and any additionally required surface treatment is crucial.

Material selection by intended application

Linear shaft material selection is based on the specific requirements of the intended application. In many cases, unhardened steel, e.g. EN 1.1191 Equiv., can be used for simple applications, e.g. any that use maintenance-free plain bearings. Additional hard chrome plating improves the surface finish and surface resistance. LTBC coatings can improve corrosion resistance. The shaft should generally be significantly harder than the plain bearing.

For linear ball bearings or higher precision requirements, harder or induction hardened steel should be used. The linear shaft and linear ball bearings should then have the same hardness. The precision steel CF53 or EN 1.1213 can also be used in this case. This steel is unalloyed and suited for induction and flame hardening. Thanks to its medium carbon content, it can be accurately machined, which is advantageous, including when high accuracy requirements need to be met

The selected material should combine the following main properties according to the weighting required by the application:

• High material strength that allows for lighter weight
• Hardenability or Hardness
• High ductility - low notch sensitivity

Certain considerations should be made to combine optimal performance, service life and efficiency:

• What are the environmental conditions? Is stainless steel needed?
• What types of loads occur (important for material hardness)?
• What surface hardness is needed? Is the relevant item for example a linear ball bearing or a plain bearing bushing incl. transported load?
• Is the required precision available?
• What is the cost?
• What manner of assembly is used? This may be relevant for notch sensitivity.

If hardness and wear resistance are the main focus, hardened steel grades such as steel with material number 1.3505 or steel with nitrated surfaces should be used. These steel grades also withstand intense use and mechanical wear & tear.

Although the shape of the shaft ends is not a selection criterion for the linear shaft material, it does affect the surface hardness of the hardened areas. Considerations for existing mounting options and resulting shaft ends make sense, especially when specialized functions are required, e.g. linear shafts with threaded studs.

Hardened vs. non-hardened linear shafts

Hardened steel shafts should be used for high precision or higher bearing requirements. Such linear shafts, also called precision shafts, are usually heat treated (induction) hardened steel shafts with a ductile core. Alternatively, a special coating can be applied for some applications (e.g. hard chrome), which increases the surface quality and hardness.

Hardened linear shafts are less susceptible to abrasion and surface deformation. This benefits them especially when they are subjected to high loads. Linear ball bearing witness marks are smaller because the hard shaft surface better withstands the stresses caused by the balls in the linear ball bearing. However, it is important to note that the hardness should not be too high since the shaft can otherwise also become brittle and fail.

Non-hardened shafts are by contrast softer and less notch sensitive, but more susceptible to wear and deformation. Non-hardened linear shafts are usually less expensive than hardened shafts.

Effect of shaft hardness on nominal useful life

The nominal useful life of an entire linear system is also influenced by the shaft hardness f_H, among other influencing variables. A shaft must be hard enough to withstand the ball bearings. The rated load is otherwise reduced. Other influencing variables include the temperature coefficient f_T, the contact coefficient f_C and the load coefficient f_W.

Temperatures above 100°C result in reduced hardness and thus a reduction of the rated load. The contact coefficient takes into account the fact that the rated load changes by the number of linear bearings per axis (linear shaft). Typically, two parallel linear shafts are installed in linear shaft guides.

The following contact coefficients f_C respectively apply depending on the number of bearings:

• One bearing per shaft: 1.0
• Two bearings per shaft: 0.81
• Three bearings per shaft: 0.72
• Four bearings per shaft: 0.66
• Five bearings per shaft: 0.61

The load coefficient f_W requires information on material weight, load torque, and other parameters that are usually difficult to calculate. The following values are used as a rule of thumb for applications without significant vibration and shock loads:

• Low speed (maximum 15 m / min): 1.0 ... 1.5
• Medium speed (maximum 60 m / min): 1.5 ... 2.0
• High speed (above 60 m / min): 2.0 ... 3.5

Together with the dynamic load rating C and the payload P, the nominal useful life L of a linear ball bearing can be calculated as follows:

Hardness of various steels

Depending on the composition and heat treatment, steel grades can range from soft, malleable qualities to extremely hard, wear-resistant variants. The material number indicates the hardness of steel:

• EN 1.3505 (100Cr6): Classic roller bearing steel with high hardness, suitable for heavy wear
• EN 1.4125 (X105CrMo17): A martensitic chrome steel with very high wear resistance, including use as knife steel
• EN 1.1191 (C45): is an unalloyed quality steel or carbon steel.  It can only be hardened moderately, but is readily machinable. Used for shafts with medium to high mechanical requirements.
• EN 1.4301 (X5CrNi18-10, AISI 304): A nickel-chromium steel. Used frequently and easy to machine. It has high corrosion resistance. The hardness is below 215 HB and hardening by heat treatment is not possible.
• EN 1.4037 (X65Cr13): A martensitic stainless steel. After hardening, it has high hardness, but is relatively brittle. It is suited for use in corrosive environments.
• EN 1.1213 (Cf53): An unalloyed high-carbon quality steel. Very good hardening properties, high strength and toughness, but diminished corrosion resistance.

See the following table, Hardness and Available Surface Treatment by Shaft Material, for the associated ISO tolerances:

Treatment of Linear Shafts and Their Effects

Linear shafts are first inductively, thermally hardened. This hardening step is completed on the raw material on the edge layer before all further machining processes. The resulting hardening depth depends on the material and linear shaft diameter. The shaft is then machined by grinding, drilling, etc. In these areas, the hardened edge layer is also removed. The surrounding material often gets very hot as a result of machining, which leads to a change in hardness in these areas.

The following table provides an overview of the hardening depth of linear shafts for various steel grades:

Hardness restrictions in surface treatment

Before machining, the steel is often heated to make it more machineable. Even by machining, steel in the edge region can be heated to such an extent that the hardness of the originally uniformly hardened edge layer in this region is reduced. This zone is also called the heat dissipation zone and has a lower hardness than the rest of the material. The heating process should be controlled to minimize the incidence risk in this zone. The hardened edge layer of the original shaft is removed on wrench flats, spigots, etc. The processed or exposed surfaces therefore exhibit a different hardness.

For example, annealing can result in reduced hardness in the following configuration options and shaft designs:

• Threaded shafts
• Stepped shafts
• Circlip grooves, conical and hex bores, wrench flats, pilot holes with internal threads, grooves for mounting screws
• Keyways, V-grooves
• Flat surfaces
• Configurable shaft end designs (G-, H-shape)
• Hollow shafts (lateral hole on one side)

Other Forms of Surface Treatments

In addition to hardening the linear shaft itself, coatings can also be applied to improve hardness. These are also used for corrosion protection. There are several types of coatings:

• Hard Chrome Plating: Hard chrome plating provides high surface hardness and wear resistance. However, chrome can chip off.
• LTBC plating: This coating is a 5μm thick layer of fluoropolymer deposited as a black layer. It is low-reflection and resistant to burst pressure by bending the linear shaft. LTBC coatings exhibit a good balance between hardness and elasticity.
• Electroless nickel plating: Uniform, pore-free layer with high corrosion protection. This coating creates a smooth surface with low friction, but only moderately increases surface hardness, which is why it is primarily used for corrosion protection and sliding properties.
• Nitriding: Nitriding significantly increases surface hardness. This process diffuses nitrogen into the steel surface. Similar to the LTBC plating, the nitriding layer can no longer chip.

MISUMI Linear Shaft Selections

MISUMI offers a variety of linear shaft configuration options:

• Shaft material: Steel, stainless steel
• Coating/plating: uncoated, hard chrome-plating, LTBC coated, electroless nickel-plated
• Heat treatment: untreated, inductively hardened
• ISO tolerances: h5, k5, g6, h6, h7, f8
• Precision classes: perpendicularity 0.03, concentricity (with thread and increments) Ø0.02, perpendicularity 0.20, concentricity (thread and stepper) Ø0.10
• Straightness / roundness: depending on the diameter. Refer to the shaft precision standards for more information.

Also read our article Linear Shafts: Precision Standards of MISUMI Linear Shafts.

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