When drives need to move very heavy equipment and withstand shock loads, designers usually choose engineering class drive chain to do the job.
Built to withstand the most rugged operating conditions, engineering class chains offer high load capacities along with the fatigue strength needed to stay in operation for long periods of time.
The basic types of engineering class chain are broadly classified as drive chain and conveyor chain. Most conveyor chains are custom designed for use in material handling operations. Drive chains, the focus of this article, generally meet industry (ANSI) standards, and they transmit power between driving and driven machines.
Engineering class drive chains frequently operate around-the-clock, and are often subjected to mud, sand, metal fines, and other abrasives. These workhorses of power transmission offer maximum working loads up to 37,000 lb. Plus, they handle intermittent shock loading, and operate with minimal maintenance and lubrication.
Applications exist in many heavy industries including food processing, earthmoving, lumber, mining, petroleum, pulp and paper, shipbuilding, steel, rubber and plastics, and waste treatment.
These chains transmit power to a variety of equipment including agitators, compressors, crushers, dredges, elevators, fans, hoists, rotary kilns, machine tools, rotary mills, mixers, oil wells, pumps, and screens. They are often used to turn heavy rotating drums, such as barking drums in pulp mills, ball mills in ore processing, or shakeout drums in foundries. They also power large off-road cranes and draglines, Figure 1.
By understanding the basics of engineering drive chain, designers can select the best type for a given application so it will operate for long periods with minimum downtime.
Engineering class drive chain consists of a series of links that are assembled by inserting pins and bushings between pairs of metal plates called sidebars. These connected links form a continuous chain that transmits power, generally from a small driving sprocket to a larger driven one on parallel shafts.
The two most common chain types, straight and offset, are identified by the shape of the sidebars that form the links, Figure 2. Other types of engineering chain having limited use in drive applications include steel bushed rollerless chain and welded steel mill chain. Steel bushed chain is used primarily for bucket elevators. Welded steel chain is a variation of offset chain made by welding bushings between the sidebars rather than press-fitting them. Welded steel chain is used mainly in wood, pulp, and paper applications.
Straight sidebar chain consists of alternate connections of pin links and bushing links, Figure 2. Each bushing link contains a pair of sidebars connected by two press-fit bushings, whereas each pin link is connected by two pressfit pins. The pin and bushing links join in alternating fashion, with the pin from one link fitting inside the bushing of the adjacent link so that each link flexes in one plane.
Straight sidebar chains accept attachments and are used primarily in conveyor applications. However, some drive applications preclude the use of offset sidebar chain because they require a short pitch (distance between pin holes in sidebars) or thick sidebars that cannot be offset. Where this is the case, straight sidebar chains are used for drive applications.
Offset sidebar chain is generally preferred for high-load applications because of its ease in adjusting length. Unlike straight sidebar chain, every link of offset chain is alike, with one pin and one bushing connecting a pair of bent sidebars, Figure 2. The pin of one link fits inside the bushing of the next link. Pins are press-fit into the sidebars and held in place with cotter pins.
Most chain drives can be adjusted to spread the shafts farther apart to compensate for chain elongation, though, eventually it may be necessary to shorten the chain. Because all links of an offset chain are the same, a single link can be removed to shorten the chain and compensate for elongation due to pin-andbushing wear. By contrast, links of straight sidebar chain must be removed in pairs.
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Pin and bushing wear depends on how the offset chain articulates around a sprocket, which in turn depends on orientation of the link. Wear tends to be less when the narrow end of the link faces the small sprocket. As a result, the chain doesn’t elongate as fast as straight sidebar chain.
Most drive chains have hardened (42 Rockwell C) steel rollers fitted over the bushings so the rollers contact the sprocket teeth and act as bearings to reduce friction and wear. Engineering class chain equipped with rollers is similar in design to conventional roller chain. However, the two types of chain differ in size, strength, tolerances, and ability to operate under adverse conditions. Engineering class chains generally carry higher loads, but at lower speeds.
Before selecting chain for a drive application, you need to know the power source (electric motors in most cases), the type of equipment to be driven, horsepower to be transmitted, speeds (rpm) of the fast and slow-speed shafts, shaft diameters, center-to-center distance between shafts, space limitations, and any unusual conditions, such as severe abrasion or corrosion. Using this information, along with industry or manufacturer’s chain capacity ratings, you can choose an appropriate chain for the application.
Here are the basic steps for selecting a drive chain.
1. Determine the load class (uniform, moderate shock, or heavy shock) of the driven equipment from the chain manufacturer’s catalog.
2. Select a service factor, ranging from 1.0 to 1.7, from the catalog. The size of this factor depends on the load class and type of power source.
3. Calculate design horsepower by multiplying the horsepower to be transmitted by the service factor.
4. Using a selection chart, Figure 3, choose a chain based on design horsepower and speed (rpm) of the small (drive) sprocket. The intersection of the design horsepower line and the rpm line indicates the recommended ANSI number or manufacturer’s chain number. Then, from the manufacturer’s catalog, find the pitch for the chain selected.
5. Determine the number of teeth for both small and large sprockets. Find the number of teeth for the small sprocket in the manufacturer’s horsepower rating tables based on design horsepower rating, speed (rpm), and pitch. Table 1 gives horsepower ratings for a 4.5-in. pitch, offset chain (ANSI-3618). Then calculate the number of teeth for the large sprocket from:
N = rn/R
N = Number of teeth in large sprocket
r = Speed of small sprocket, rpm
n = Number of teeth in small sprocket
R = Speed of large sprocket, rpm
6. Determine the minimum center distance between sprockets, using the formula:
C= (2N+ n)/6
C = Minimum shaft center distance in pitches
N= Number of teeth in large sprocket
n = Number of teeth in small sprocket
This formula gives the minimum distance. The final value may be varied slightly to suit the space available and to provide sufficient space for the sprockets.
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7. Calculate the chain length:
L = S/2 + 2C + K/C
L = Chain length in pitches
S = Total number of teeth on both sprockets
C = Shaft center distance in pitches
K = A value related to the difference in number of teeth between the small and large sprockets, obtained from a manufacturer’s table.
To convert chain length to feet, multiply the length in pitches by the pitch in inches divided by 12.
For chains not shown in Figure 3, an alternate method of chain selection involves calculations from working load values for a chain taken from manufacturers’ specifications. Consult the manufacturer for the formula and assistance in determining whether a particular chain is suitable for a specific application.
Ensuring chain durability and long life
The test of an engineering class drive chain comes when it is operated under severe conditions with little or no lubrication. But don’t wait until then to find out if it’s up to the task — evaluate the chain before you buy. Here are some things to look for:
Materials and heat treatment. Use alloy steel sidebars and bushings, throughhardened to 375 BHN for strength and wear life, especially for larger size chain. Most designers choose steels such as AISI 4130 or 4140 for harsh environments. Alloy steel pins should be induction hardened to 55 Rockwell C for wear resistance. Use hardened teeth (minimum 35 Rockwell C) for sprockets with less than 15 teeth, speeds over 600 rpm, speed reduction ratios over 4:1, high loading, and abrasive environments.
Pitch control. Controlling the chain pitch avoids excessive wear due to mismatch between chain and sprocket. Pitch is controlled by positioning of sidebar holes made by punch presses, so punch press tooling must be accurate.
Pitch holes in sidebars. Sidebar hole diameter must be maintained through entire depth of the hole. Some punch presses tear material from inside the hole, causing a flare at one end.
Bushings. Smooth bearing surfaces ensure low resistance to motion and long life. Bushings should be precision machined and pins should be finely ground to match the bushings into which they will be inserted.
Edwin Pawlicki is manager of engineering, Union Chain Div., U.S. Tsubaki Inc., Sandusky, Ohio.