To catch up on part 1, read the post here.
You would not be blamed for mistaking linear shafts, rotary shafts, and posts at first glance. All three have similar appearances as they are all generally metallic, rod-shaped assembly components, that are visibly sturdy as they are all meant to handle loads. They all also offer similar machined features such as wrench flats or threads. However, these three are in fact different types of shafts that have different uses and complementary components. Keep their use cases in mind when choosing between them for your application.
Linear shafts provide support and guidance to linear load transport applications. They are often paired with linear bushings which ride along the length of the shaft. The two typical methods of linear shaft mounting are continuous support and end-supported. In most cases, Linear shafts with continuous support are used for heavier loads, while smaller loads can be supported at both ends instead. Linear shafts may also be made hollow to reduce weight in the assembly.
Rotary shafts are similar to linear shafts but as the name implies, they are designed to rotate about a longitudinal axis, transmit torque and withstand torsional forces. They are commonly paired with rotary bearings and can be assembled into pulley systems, use chains, or can act alone when coupled to a motor. Drive shafts are also similar to rotary shafts but the biggest difference between them is that drive shafts are precision components and therefore have tighter tolerance standards. Drive shafts are generally recommended for use in high-speed applications.
Although posts (standoffs) also appear similar they do not facilitate linear nor rotary motion. Instead, they are structural features that are used to statically support loads and position other components in assemblies. Because posts and their mating components are not in motion, tolerances are less critical and they are therefore the most inexpensive of the three. They come in different forms such as round, hex, or square shapes.
All three of these components are commonly machined to include features or alterations such as axial or radial holes, keyways, snap ring grooves, or stepped or chamfered ends depending on the mounting method that is going to be used. Male or female threads are also common for fastening shafts to support assemblies. In addition, there are also a variety of options for material, heat treatments, surface finishes, and fit tolerances as shown below.
| Linear Shafts | Rotary Shafts | Posts | |
| Surface Treatment | Induction Hardening | ||
| Surface Finish | Electroless Nickel Hard Chrome Plating Low-Temperature Black Chrome | Black Oxide Electroless Nickel | Clear Anodized Black Anodized Black Oxide Electroless Nickel |
| Material | Aluminum Bearing Steel Stainless Steel | Bearing Steel Stainless Steel | Aluminum Bearing Steel Stainless Steel |
Heat Treatment
Induction Hardening
The common practice in bearing steel technology is to induction harden the steel to increase the hardness of the surface, thereby withstanding the wear of the contacting surfaces.
Material
Bearing Steel
Provides the highest durability at the cost of susceptibility to corrosion.
Stainless Steel
Well-suited for applications in which corrosion resistance is important. Also suitable for food applications, clean room environments, or erratic temperatures.
Aluminum
Resistant to oxidation as well as provides good electrical and thermal conductivity, and high reflectivity.
Surface Finishes
Hard Chrome Plating
Offers excellent wear resistance to steel along with increased abrasion resistance.
Check out our surface finishes blog on hard chrome plating at this link.
Low-Temperature Black Chrome
Offers exceptional corrosion resistance while maintaining a very high precision tolerance. This coating has a thickness of about 1-2 microns.
Learn more about low-temperature black chrome plating in our blog here.
Black Oxide
Offers mild corrosion resistance and prevents light reflection. This coating does not alter the dimensions and does not peel or chip.
Electroless Nickel
Offers excellent corrosion resistance as well as additional hardening for better wear resistance.
Read more about electroless nickel plating via our blog post here.
Black or Clear Anodize
When used to finish Aluminum, it improves its resistance to corrosion and wear.
More about anodizing can be found on our blog post at this link.
Shaft Tolerance and Fits
Check out our blog post on shaft tolerances here.
Clearance Fit
Ensures a shaft can be freely inserted into the intended bore. This generally means the maximum limit of the shaft is less than the minimum limit of the bore diameter.
Interference Fit
Guarantees the shaft and bore will interfere at every point within their tolerance zone and is commonly referred to as a press fit. Often the two components must be joined with excessive force or by altering the atmospheric temperature to slightly expand or shrink the diameter for assembly.
Transition Fit
A combination between the clearance and interference fits. Depending on the actual dimension of the shaft and bore it is possible that either can occur. This fit is best used when a shaft must be held in a precise location.
Be sure to configure your linear shaft, rotary shaft, or post with the surface treatment of our choice and receive it in about 4 – 7 business days.
Phew! It was such a relief when you mentioned that black oxide does not cause the structural integrity of a material to be altered. My nephew have a few rotary shafts at his workshop which need to be maintained before the end of this week when a regular check is conducted. I’ll ask him to take a closer look into this matter before he takes any further action.