Shafts
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- Lesley Lang
- Former user (Deleted)
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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End types
Various end types for shafts as well as their benefits are listed below [4]:
| End Type | Description | Visual |
|---|---|---|
Straight | Can be mounted directly into bearings, housings, and shaft supports, or be used as a blank to machine individual end types. |
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Keyed | Combined with a machine key to transmit torque to gears, sprockets, and other power transmission components. |
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D-Profile | A flat surface area allows set screws to dig into the shaft for securely mounting gears, sprockets, and bearings. |
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Splined | Commonly used for high-torque applications, such as hydraulic systems and machine tools, with multiple keyway-like grooves. |
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With Retaining Ring Grooves | Retaining rings can be clipped into the grooves to separate and position bearings, collars, and sprockets. |
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Step Down | A shoulder (either step-down x straight end or step-down x step-down) near the end of the shaft provides a stop for shaft supports, gears, and sprockets. |
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Threaded | Mounted into tapped holes or attached threaded accessories (e.g. fan blades, propellers). |
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Tapped | Internal threads allow mounting onto threaded studs or attach threaded components. |
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Hollow | Reduce total system weight and allow running various materials (e.g. electrical wiring, compressed air tubing, coolants, lubricants) through the center. |
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Shaft Keys
A keyed joint consists of a key, keyway, and keyseat [9]. A key is a piece of metal used to connect rotating elements to a shaft by preventing relative rotation between the two parts, enabling torque transmission to occur. A shaft and the rotating element(s) (gears, pulleys, couplings) must have a keyway and a keyseat, respectively. A keyseat is also referred to as a groove or a pocket. The width and height of the key/keyway must adhere to specific tolerances in order to prevent misalignments.
Various shaft keys and benefits are listed below [10]:
| Key Type | Description | Visual |
|---|---|---|
Straight Machine | The most common type of key, usually with a square or rectangular cross section. They are often as strong or stronger than the material of the shaft or the other components being used. A softer key material is used as a sacrificial part that will shear off if excessive force is applied, preventing damage to the shaft and/or other components. |
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Rounded Machine | Similar to straight machine keys, but rounded ends are used to make them easier to slide gears, hubs, and other components into place. |
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Tapped Rounded Machine | When first installed, they are pressed into place, similar to a rounded machine key. A screw is then threaded through the tapped hole to pop these keys out of a keyway. Often used for permanent or high-torque applications. |
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Woodruff | Work well near shaft shoulders or tapered shafts. The rounded shape allows these keys to be more easily removed. Best used for light-duty applications. |
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High Profile Woodruff | Similar to woodruff keys, but feet are used to prevent the keys from rocking in the keyseat, making it easier to install gears and other components. |
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Tapered Gib Keys | The tapered body wedges into a keyway for a more secure fit than standard machine keys. The gib head makes these keys easy to remove. They are tapered along their length. |
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Design Considerations
Design considerations for shafts include: shaft diameter, sizing and spacing, tolerances, material selection/treatment, deflection/rigidity, strength, frequency response, and manufacturer restraints [2]. Shafts typically have stepped diameters to account for bearing mounts or to provide shoulders for devices like gears, sprockets, and pulleys to be supported against. Keys are often used on these devices/additions, in order to prevent rotation of the object relative to the shaft. Critical speeds are rotating speeds at which the resulted deflections become unstable. Components attached to the shaft have an even lower critical speed than the shaft itself. The lowest critical speed should be at least twice the operating speed.
Radial and Axial Loads
Radial load is the maximum force that can be applied to the shaft in the radial direction (perpendicular to the motor shaft axis) [11]. Radial load is also referred to as the "overhung load" because of how the load may "hang" off the shaft. Radial loads change by the distance between the installation point of the overhung load to its support bearing.
Axial load is the maximum force that can be applied to the shaft in the axial direction (same axis as or parallel to the motor shaft axis) [11]. Axial load is also referred to as the "thrust load" since thrust force and thrust load act upon the same axis.
Movement Constraining
External mounts can be used to constrain movement along a shaft. These include:
Figure 7: Radial and Axial Loads [11] |
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| Type | Description | Visual | ||
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Snap Ring | They are axially installed and used externally to secure parts on a shaft or in a bore [12]. The protruding section of the ring provides a shoulder to position retaining parts, which avoids the need to machine shoulders. Also known as retaining rings, they can be internal or external. Internal retaining rings can fit into a housing or bore while external retaining rings fit over a shaft or pin. There are many types of retaining rings such as grooveless retaining rings which do not require grooves to be placed on. Another type of retaining ring is the spiral retaining ring, which are coiled, flat, and have no gaps or protruding "ears". |
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Dowel Pin | Dowels can be used to keep machine components in accurate alignment on shafts, locking their linear motion[13]. Standard dowel pins are similar to shafts themselves. A tight fit of a machine part to a shaft via a dowel should use a hole size that is equal to or slightly smaller than the dowel size. |
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Cotter Pin | Cotter pins (also known as split pins) have two prongs, one slightly longer than the other, which makes them easy to open [14]. After inserted into a shaft (or a clevis pin), the prongs are then bent outwards to secure the component. They are used in combination with clevis pins to keep components in place on shafts. |
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Clevis Pin | Clevis pins (combined with a cotter pin) are common alternatives to bolts and rivets [15]. After the clevis pin is inserted to hold a mechanical component to the shafts, a cotter pin is placed through the hole of the clevis pin and the prongs are bent outwards to secure. Together, they restrict a component's linear movement along the shaft. |
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References
[1] Componex, "WINshaft Hard Chrome Plated Shafts," [Online]. Available: http://www.componex.net/tech-info/winshaft-hard-chrome-plated-shafts/. [Accessed 2 February 2021].
[2] P. R. Childs, "Shaft Design," Mechanical Design Engineering Handbook, 2014, vol. 1, no. 1, 2014.
[3] ThomasNet, "Types of Shaft Couplings - A Thomas Buying Guide," [Online]. Available: https://www.thomasnet.com/articles/hardware/coupling-types/. [Accessed 22 January 2021].
[4] Phidgets, "17mm Precision Ground Rotary Shaft (no keyway)," [Online]. Available: https://www.phidgets.com/?tier=3&catid=83&pcid=74&prodid=747. [Accessed 25 February 2021].
[5] McMaster-Carr, "About Shafts," [Online]. Available: https://www.mcmaster.com/shafts. [Accessed 11 February 2021].
[6] Engineering 360, "Spline Shafts Information," [Online]. Available: https://www.globalspec.com/learnmore/mechanical_components/power_transmission_mechanical/spline_shafts. [Accessed 10 March 2020].
[7] Misumi Mech Lab, "Shaft and Hole Tolerances and Fits," [Online]. Available: https://blog.misumiusa.com/shaft-hole-tolerances-for-clearance-interference-fits/. [Accessed 11 February 2021].
[8] China Composites, "Factory Wholesale Carbon Fiber Transmission Shaft," [Online]. Available: https://www.china-composites.net/carbon-fiber-product/carbon-fiber-cooling-shaft/carbon-fiber-transmission-shaft.html. [Accessed 11 February 2021].
[9] LoveJoy by Timken, "Why Power Transmission Shafts Have Both Keys and Keyways," [Online]. Available: https://www.lovejoy-inc.com/resources/technical-articles/why-power-transmission-shafts-have-both-keys-keyways/. [Accessed 25 February 2021].
[10] McMaster-Carr, "Shaft Keys," [Online]. Available: https://www.mcmaster.com/shaft-keys/. [Accessed 25 February 2021].
[11] Oriental Motor, "Motor Sizing Basics Part 4 - Radial Load and Axial Load," [Online]. Available: https://blog.orientalmotor.com/motor-sizing-basics-part-4-radial-load-and-axial-load. [Accessed 25 February 2021].
[12] Engineering 360, "Retaining Rings and Snap Rings Information," [Online]. Available: https://www.globalspec.com/learnmore/mechanical_components/mechanical_fasteners/retaining_rings_snap_rings. [Accessed 10 March 2020].
[13] McMaster-Carr, "Dowel Pins," [Online]. Available: https://www.mcmaster.com/dowel-pins/dowel-pins-7/. [Accessed 10 March 2020].
[14] McMaster-Carr, "Cotter Pins," [Online]. Available: https://www.mcmaster.com/retaining-pins/cotter-pins-5/. [Accessed 10 March 2020].
[15] McMaster-Carr, [Online]. Available: https://www.mcmaster.com/retaining-pins/clevis-pins-7/. [Accessed 10 March 2020].
Contributors:
| User | Last Update |
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| Lesley Lang | 1120 days ago |
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