Updated July 24, 2026.
Linear shafts, posts, and rotary shafts are all cylindrical components found in machine assemblies—but they serve fundamentally different purposes. This guide explains how each works, what type of motion each supports, real-world use cases by industry, and a simple framework for selecting the right one for your application.
Linear shafts, posts, and rotary shafts are used when you need to control some type of motion: linear, rotary, or a combination of the two. Before we dive into the motion aspects, let’s take a look at how each option works.
Linear shafts
The term “linear shaft” can be a little misleading, as the shaft isn’t actually doing any work—it’s just there for support. A linear shaft is used when a sliding motion is needed, especially when that motion needs to be guided and fine-tuned. The shaft acts as a precision track along which a carriage, bushing, or bearing block slides. An actuator (pneumatic, electric, hydraulic, or manual) provides the driving force; the linear shaft and its mating sliding guides constrain the motion to a straight, accurate path. The example above shows the actuator doing the work of lifting the load and the linear shafts and bushings supporting.
Shaft diameter and tolerance class are determined by the applied load, required travel precision, and deflection limits./p>
- ▸Motion type: linear (sliding / translating)
- ▸Active or passive: passive—guides motion, does not drive it
- ▸Mating components: linear bushings, linear bearings, shaft supports, end supports
- ▸Sizing driver: applied load, deflection limit, required precision
- ▸Typical materials: carbon steel (hard chrome plated), stainless steel
Linear shaft use cases
Posts (standoffs)
A post, also called a standoff or strut post, is exactly what it sounds like: a cylindrical bar fixed at one end to a base that has something that rotates around it. Its purpose is to position, separate, or support components within a machine assembly at a defined height or spacing. The above example shows hex posts being used in an assembly.
Posts are available in round, hex, and square cross-sections. Hex and square posts provide a wrench flat, making them easy to install and torque without slipping. They can serve as pivot points for components that need to rotate around a fixed axis (like an idler pulley or a hinged guard), as spacers between structural plates, or as adjustable height standoffs for mounting brackets, sensors, and other accessories.
- ▸Motion type: none (structural) or rotation-around-post as fixed pivot
- ▸Active or passive: fully passive—structural and positioning only
- ▸Available cross-sections: round, hex, square
- ▸Sizing driver: required height/spacing, load in compression or bending, thread specification
- ▸Typical materials: carbon steel, stainless steel, aluminum
Post use cases
Rotary shafts
A rotary shaft transmits torque and power from a motor or drive source to another component. Unlike a linear shaft, it is an active member—it rotates and does mechanical work. It can operate alone (like an automotive driveshaft) or as part of a system with pulleys and belts, sprockets and chains, or gearboxes. The above example shows a rotary shaft doing the work by turning and transmitting power.
Shaft sizing is determined primarily by the amount of power that needs to be transmitted. Key design considerations include keyway geometry, surface finish at bearing seats, shaft deflection under load, and critical speed at the operating RPM. For applications coupling a rotary shaft to a motor, see the related guide on sizing timing belts and pulleys.
- ▸Motion type: rotary (torque transmission)
- ▸Active or passive: active—transmits power and torque
- ▸Mating components: couplings, bearings, pulleys, sprockets, gears, keyways, set screws
- ▸Sizing driver: transmitted torque, bending load, deflection, critical speed
- ▸Typical materials: carbon steel, alloy steel, stainless steel
Rotary shaft use cases
Side-by-side comparison
| Property | Linear Shaft | Post / Standoff | Rotary Shaft |
|---|---|---|---|
| Primary function | Guide sliding motion | Position / space components | Transmit torque & power |
| Motion type | Linear (translating) | None (or fixed-axis pivot) | Rotary |
| Active or passive | Passive | Passive | Active |
| Rotates? | No | No (component rotates around it) | Yes |
| Transmits power? | No | No | Yes |
| Sizing driver | Load, deflection, precision | Required spacing, compressive/bending load | Torque, bending load, critical speed |
| Typical cross-section | Round (precision ground) | Round, hex, or square | Round (with keyway or spline) |
| Common mating parts | Linear bushings, shaft supports | Brackets, bearings, spacers | Couplings, pulleys, gears, bearings |
How to choose: define your motion first
As with a lot of engineering problems, identifying the type of motion is the most critical aspect of the design. Once you have that knowledge, choosing between linear shafts, posts, and rotary shafts is quite easy.
Sizing considerations by type
Linear shaft sizing
Select diameter based on the applied load and the acceptable deflection over the unsupported shaft span. A longer unsupported span dramatically increases deflection for a given load—doubling the span increases deflection by a factor of eight for a simply supported beam. For applications requiring higher precision, choose a tighter tolerance class (h5 or h6). MISUMI linear shafts can be configured in 1mm length increments with a range of end treatments at time of order.
Post sizing
Posts in compression are sized by the required height and the load they must carry without buckling. Posts subject to bending (e.g., a cantilevered bracket mounted to a post) require attention to bending stress at the base. Hex posts offer a practical advantage for installation: the flat surfaces allow wrench tightening without a separate fixture. Round and square posts are available where cross-section geometry is a design constraint.
Rotary shaft sizing
Shafts transmitting torque are sized using the torsional shear stress equation, with a safety factor applied for shock loads and fatigue. Bending from overhung pulleys, gears, or sprockets adds to the stress at critical sections. At high speeds, verify that the operating RPM is below the shaft’s critical speed to avoid resonance. Keyway cuts and cross-drilled holes are stress concentration points that reduce fatigue life and must be accounted for in the sizing calculation.
