Shafts
Table of Contents
Figure 1: Various Shafts [1] |
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What are Shafts?
Shafts are components of circular cross section that transmit power and rotational motion from a driving device, such as an engine, through a machine [2]. Mechanical elements, such as gears, pulleys, flywheels, clutches, and sprockets are often mounted to various types of shafts. There are two main types of shafts: transmission shafts are used to transmit power from a source to the machine and machine shafts are integral parts of the machine itself. Shafts are often connected via shaft couplers, which also absorb misalignments over time [3].
Types of Shafts
Rotary Shafts
Rotary shafts are often working with gears, sprockets, and bearings to transmit rotary motion [5]. Rotary shafts are generally not hardened, which makes them easier to machine and fit mating components. They are also commonly referred to as drive shafts.
Linear Shafts
Linear shafts are used for sliding motion, especially when that motion needs to be guided and precise [5]. The shaft size and precision are dictated by the load and requirements of the motion needed.
Linear motion shafts are smoother, harder, and more wear resistant than rotary shafts. They often work with linear bearings to reduce friction in various operations. The smooth surface reduces friction and wear on the bearing. A shaft's surface smoothness is measured in a value called micro-inches. Lower micro-inch values correspond to smoother finishes and less overall friction.
Linear motion shafts are typically hardened for increased wear resistance. Case-hardened shafts are hardened only on the surface of the shaft, which increases wear resistance while allowing the center to remain soft for absorbing stresses caused by shifting loads.
Spline Shafts for Rotary and Linear Motion
Spline shafts are often used for rotary and linear motion and transmit rotary torque while allowing the bearing to move linearly along the shaft [5]. They are typically used for robotic systems and other complex automated movements.
Spline shafts lock rotation for bearings or bushings when functioning as a guide for linear motion [6]. The splines (grooves) are machined along the length of the shaft. A female-splined bore, gear or bearing can be mated to a spline shaft to either allow rotational torque in drive applications, or linearly guide the mounted components along its length.
There are a few different types of spline shafts. Parallel key splines have squared ridges while involute splines contain tapered ridges for decreased stress concentration. Helical splines (either parallel or tapered) have ridges that form a helix pattern about the shaft, which allow for rotary and linear motion while minimizing stress for a stationary joint with high load.
Figure 2: Rotary Shaft [4] |
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Figure 3: Parallel Key Spline Shaft [5] |
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Tolerances and Fits
The majority of mechanical designs involve shaft and hole joint, along with which are various tolerances and fits [7]. The two main fit designations are clearance fits and interference fits, but many fits lie on a spectrum between the two, denoted as transition fits. A clearance fit specifies a fit where there will always be a gap in the joint between the mating shaft and hole, allowing free movement for the shaft through the hole. The maximum shaft tolerance and minimum hole tolerance (i.e. largest shaft and smallest hole) should still permit the shaft to freely pass through the mating hole. An interference fit, on the other hand, is a fit where there will always be overlap in the joint between the specified mating shaft/hole. This means force or temperature fluctuation will be required to get the shaft into the hole. The minimum shaft tolerance should still allow the overlap if this fit is specified. A transition fit lies between a clearance fit and interference, and is best used for a shaft that must be held in a precise location.
Clearance Fit and Tolerances | Interference Fit and Tolerances | ||||
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More information on dimensioning and tolerancing can be found here.
Material Selection
Various manufacturers have wide selections of materials, each catering to a slightly different intended use. Some of the most common materials are listed in the table below, including their different benefits [5].
Shaft Material | Benefits |
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Carbon Steel | Balances high strength and good machinability, making them ideal for general purpose use. |
Alloy Steel | Harder and stronger than carbon steel and provides superior durability. |
Stainless Steel | Used for applications in which corrosion resistance is an important consideration. |
Aluminum | Provides good electrical and thermal conductivity, high reflectivity, and resistance to oxidation. |
Composites | Often made of carbon fiber bonded by resins; they are lightweight and help reduce energy requirements. |
Figure 6: Carbon fiber composite transmission shaft [8] |
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