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Proper Shaft Fits for Precision Drive Components

When sizing high performance drive components, proper shaft fit is an important consideration in helping to maintain concentricity and balance in the rotating equipment. In v-belt drives, chain drives and other low-cost / low-speed systems, designers often give little thought to shaft fit. Many manufacturers of these types of low cost drive components are simply relying on a shaft key and set screw to transmit torque and retain the hub on the shaft, and other types of misalignment absorbing elements help to take up any eccentricity resulting from a relatively large clearance between the shaft and the bore.  But when it comes to high-performance / high-speed drives and “smart” motors, such as servo and steppers, shaft fit becomes increasingly important.

When researching a possible vendor for zero backlash components with engineered clamping systems that can be assembled by hand, it is imperative that they are able to provide a shaft tolerance as well as a published chart which verifies the shaft and bore fits required. As a general rule of thumb, the overall diametrical clearance between the shaft and the bore should be around 10-50 µm (.0004-.0019”) for smaller diameter shafts, and as shaft diameters increase beyond 80mm (3.150”), larger clearances become allowable, and more annulus is required around the ID of the bore to ensure easy installation. If the fit is correct, a bit of oil is all that is required to easily slide a hub onto a shaft. 

 Many engineers are inclined to specify interference fits for high performance drives, which in a solid hub design would require either sub-zero cooling of the shaft and / or, more commonly, heating of the bored and keyed hub. This type of fit dates back to times before frictional clamping systems became well established in the power transmission industry, and is intended to eliminate problems associated with loose shaft fits and backlash.  Requiring this type of work can create a maintenance and installation problem, and is not necessary in most modern applications incorporating engineered clamping systems. But the importance of precision shaft fit remains when it comes to frictional clamping style hubs. 

Frictional Clamping Hubs

In the case of clamping collarstyle hubs, which offer the advantage of very easy installation and removal, if the fit clearance is too large between the shaft and bore, once clamped, the hub can become eccentric to the shaft.  This can cause an imbalance and or misalignment issue in addition to the potential for failure of the driveline. In self centering clamping systems, often used for high speed / high power systems, excessive fit clearance can reduce the transmittable torque of the shaft hub connection.  But with proper shaft fits and screw tightening torques applied, they offer superior performance to simple keyway hubs, and far easier handling than interference fit hubs. 

 (clamping collar)        (self-centering clamping system)

Manufacturing Standards

There are several standardized shaft tolerance charts commonly used in industry such as ANSI B4.1-1967 (R2004) for imperial units and ISO tolerance system per DIN 7160 (8.65) for metric units.  Precision drive components should have a clearance fit which is just out of range of a transition fit such as an H7/h6 fit per DIN 7160 (8.65) chart. For example on a 32 mm diameter shaft, this would mean that the shaft could be 0-16 µm under 32mm.  The bore could be 0-25 µm over 32 mm. This would result in a diameter difference of 0-41 µm (.0000-.0016”).

If the shaft and bore were both the same size, assembly would be difficult. In practical manufacturing, this would almost never happen. A good machinist would normally use a H7 go/no-go plug gauge and an h6 go/no-go ring gauge. The go portion of the plug gauge would not practically fit into the bore by hand unless the bore was slightly larger than 12 mm exactly. Vice-versa, with the ring gauge and shaft, when checking a turned part with a ring gauge, it would not fit by hand unless the shaft was a bit smaller. If either no-go plug or ring gauge fits, the parts are scrapped. This method of machining/checking shafts and hubs results in parts with a precise fit which can be assembled by hand. 

For more information on proper shaft fits and precision couplings for high performance drive applications, contact applications@rw-america.com.

Configurable coupling systems save time and money

Many machine builders are well aware of the advantages of using configurable components in the design stages. As projects progress, specifications often change and builders must adapt components to fit new constraints. Drive and electrical components must be flexible to adapt to new operating parameters or machine chassis dimensions. Electrical designers often make use of DIN rail mounted devices which can be interchanged quickly on the production floor or in the field. Circuit breakers, fuses, and power supplies can be easily changed out in the event that a different motor or sensor is installed. From the mechanical perspective it is usually coupling elements which need to be changed to adapt to new dimensions.  This is why QD or similar bushings are typically used with V-Belt sheaves.  If drive speeds change, a fairly quick sheave swap is all that is required. If a new gearbox or actuator needs to be installed, drive couplings in various lengths and bore diameters are often required in order to help get everything tied together.  Finding direct drive couplings with compact dimensions and creative mounting configurations is normally fairly easy.  But in some cases layout changes involve longer distances between mechanically connected equipment, requiring something a little more specialized.

 Belt drive or direct drive

With longer distances between rotating components, designers are often in a position to choose between belt or chain driving, and using a direct drive line shaft system.  A direct drive line shaft coupling typically provides for stiffer power transmission than belts, which can be especially advantageous in applications that require precision timing and positioning or frequent changes in rotational direction. Line shaft couplings are also low maintenance compared with belts which need to be changed at regular intervals, just like car tires for optimal performance. But in the past there were occasionally major drawbacks to using direct drive line shafts over belts in some applications.  Assembly with steel shafting and standard couplings generally requires intermediate support bearings and is not very well suited to higher drive speeds over long distances.  Most preassembled torque tube styles of line shaft couplings are also built to order rather than being stocked by many large industrial supply companies. In the past a typical prefabricated torque tube style line shaft would need to be rebuilt if length or shaft sizes changed. 

New configurable line shaft couplings

In more recent years, prefabricated, variable length, telescoping line shaft couplings have been brought to the industrial market in order to address the growing trend toward designing with direct drives. A variable length line shaft with removable hubs solves the issue of shaft sizes and lengths changing (within adjustment range) as a machine is built or upgraded. With a variable length line shaft, the overall length can be changed in minutes by simply loosening and tightening a couple of machine screws. Jaw style hubs or flange mounted bellows coupling hubs can be swapped out with stock parts in less time than it takes to change a v-belt sheave and re-tension belts. Additionally, common size adjustable line shaft couplings are stocked by many distributors and ready to ship with hubs just like sheaves and belts.

This new take on configurable coupling component technology ultimately saves cost in labor and/or materials over time. While newer components produced in smaller quantity can have higher pricing up front, they can actually lower the cost of maintenance and overhaul down the road. Making use of a more precision product can also help to increase the rate at which a product is manufactured. As we all know, increased productivity and lower service costs ultimately decrease the time over which a return on initial investment is seen. This makes money for our employers, which is something I’ve found they enjoy universally.