Productivity is a shared goal in industrial applications, involving designers, component makers, and end users. Everyone plays a role. In this report, the editors of Motion System Design polled belt and chain drive experts for their advice on optimizing productivity. Here are the responses on both sides of the issue, which we believe you'll find most helpful.
What design/construction features in belt and chain drives CONTRIBUTE to higher productivity?
Jim/Rapid Response: Lifetime and performance in timing belts are greatly affected by the type of reinforcement employed. This internal component largely determines belt strength (modulus), creep, flex fatigue, and length variability (caused by humidity and temperature variation). Although timing belt drives are generally considered to be very efficient, the operating temperatures they are exposed to can have a significant effect. Elastomers that run well hot will generally become much stiffer at low temperatures. This reduces drive efficiency as more energy is required to bend the belt around the pulleys. Elastomers that flex at lower temperatures are available, but they often wear too quickly at mid-range and elevated temperatures.
What do designers need to know to ensure HIGHER PRODUCTIVITY from belt and chain drives?
Jim/Rapid Response: Reverse bends: Reverse bending increases the flex fatigue of the belt reinforcement fibers. If a serpentine belt path is inevitable, be sure to use idler pulleys at least 30% larger than the smallest diameter pulley in the system.
Idler pulley position: For backside idlers, the pulley should be positioned on the slack side of the belt as close to the driver pulley as possible. For an inside idler, the pulley should be placed on the slack side of the belt as close to the driven pulley as possible. If the drive is reversing, then two idlers are required because the slack side of the belt changes when the drive reverses.
Drive alignment and robustness: When designing belt drives, there is a tendency to reduce cost by sacrificing rigidity. To prevent belt wear from misalignment, pulley shafts, mountings (including idlers), and the machine chassis itself must not flex under load.
Pitch and width selection: Another tendency is to reduce cost by using under rated belts. The correct pitch and tooth profile must be selected to transmit the required load in the available space. When comparing pitches, changing from a smaller tooth pitch to a larger pitch allows the use of a narrower belt while improving belt tracking.
Belt meshing noise: Curvilinear tooth profiles tend to run quieter than trapezoidal profiles in most drives. In many cases, a narrower belt can be used when replacing a trapezoidal profile with a curvilinear one.
Small drive pulleys: As belt drive designers try to remove cost from their designs, they tend to choose high-speed, low-torque over low-speed, high-torque motors. But high-speed, low-torque motors necessitate very small drive pulleys to achieve effective belt tension. If the pulleys are smaller than the belt manufacturer's minimum recommended size, they will reduce drive efficiency because of the additional stresses generated in the belt materials as they try to conform to the small radius. They'll also reduce belt life, exposing the reinforcing fibers to severe bending stresses.
Crowned idler pulleys: Crowned idler pulleys are not a good choice for belts reinforced with high tensile helically wound fibers. The crowned face generates a high stress gradient across the width of the belt, which can cause the belt to split down the middle along its length. Flat, smooth-face pulleys with flanges are the optimum choice for idler pulleys used with belts reinforced with high-strength fibers.
What do end users need to know to ensure HIGHER PRODUCTIVITY from the belt and chain drives on their machines?
Jim/Rapid Response: Maintaining recommended belt tension is very important. If tension is too high, the drive could overload bearings and pull shafts out of alignment. It can also lead to pitch mismatch between belt teeth and pulley grooves, making belt teeth wear more quickly. Low tension can have similar consequences.
The three main methods of adjusting installation tension are fixed centers, live spring-loaded idlers, and “adjust and lock in place” Many designers opt for the low-cost approach, using fixed centers for the pulleys. But it is very difficult to achieve the correct installed belt tension with this method because of manufacturing tolerances of the belt, pulleys, and chassis. Live spring-loaded idlers can help, but if they bounce during operation, cogging will be a problem. The recommended method is to adjust one of the pulleys to apply the right belt tension, and then lock it in that position.
What are some of the SHORTCOMINGS in the design/construction of belt and chain drives, and how do they lead to lower productivity?
Eric/Optibelt: It depends on what type of belt drive you are talking about. If it is a friction drive belt, like a V-belt, then a common shortcoming is the inherent inefficiency built into the drive. On the drive's very best day it is from 97% to 98% efficient, and due to belt stretch, it goes down from there. Synchronous belt drives are more efficient and tend to be maintenance free, but are still only 98% efficient. They also have a tendency to be noisy, and are limited to specific mathematical combinations in terms of the number teeth on the pulleys and belts.
C.J./Drives Inc.: While both belt and chain drives inherently provide the same basic function in transmitting rotational motion, and both are very efficient, roller chain drives are more versatile and adaptable. They can be applied in a variety of circumstances where slight misalignment exists and/or center distances are not precise. They're also capable of operating over a fairly wide temperature range, from approximately 0 to 350°F (carbon steel construction).
In terms of adaptability, roller chain drives are suited for wet environments as well as those contaminated with dirt, dust, and grit. What's more, roller chain is available in a wide variety of standard lengths, and is easily modified to fit any and all drives in a given facility.
Dan/Gates: Chain: Inadequate lubrication: Around 75% of chain drives are not properly lubricated because of poor maintenance practices and the time required to daily hand-lubricate chain drives. High-speed, high-load chain drives often use oil bath or oil stream lubricating systems. These systems can leak, the oil has to be changed and replenished on a regular basis, and the excess, dirty oil has to be disposed of properly. The expense of lubrication and the associated downtime reduce productivity for the user. Synchronous belts, however, are maintenance-free because they do not require lubrication. Regarding service life, synchronous belt drive systems will last at least three times longer that roller chain drives.
Poor quality: We are currently experiencing a flood of inferior, imported mechanical power-transmission products. There are probably a dozen major chain and belt manufacturers who provide good, high-performance products. Design engineers who select imported products must realize, that although cheaper, these products may not provide the same service life as the products from high-quality suppliers.
Contaminated/abrasive environments: Many chain drives are open, non-enclosed systems. Chain drive lubricants attract dirt, dust, lint and airborne particles, which wear the chain components and cause elongation and wear. V-belt and synchronous drives also suffer some degradation in this type of environment, but not to the same extent as an oily chain drive.
High operating speeds: Depending on the size of the drive, chain drives generally operate best at speeds of 500 rpm or less. Synchronous drive systems can easily handle 5,000 to 6,000 rpm, making them ideal for high-speed drives and increased throughput.
Belts: Adverse environments: Like chain, V-belts are adversely affected by dirt, dust, lint and airborne particles, excessive heat, and caustic chemicals. Synchronous belts made of polyurethane (not rubber) have a greater resistance to these adverse conditions, and therefore will have a longer service life.
Slip and creep: V-belt drives are not designed for low-speed, high-torque applications. Nor should they be used on precision, positive drives because they will slip or creep (constant state of speed loss). Slippage reduces drive efficiency. V-belts are also less efficient than chain or synchronous belt drives. After installation, V-belt drives that are not retensioned or properly maintained may run at efficiencies as low as 85 to 90%. With minimal maintenance, a chain or synchronous belt drive will run at 97 to 98% for the life of the drive.
Structural rigidity: Unless a V-belt drive system has an adequate structure, bracketry or framework, the center distance between the pulleys may decrease as torque is applied to the drive. The V-belts will become loose and the necessary pre-tension will be lost. A synchronous belt drive also should be mounted on a strong framework to avoid any possibility of the belt teeth climbing on the sprockets or ratcheting.
What are some of the COMMON MISTAKES designers make when selecting or applying belt or chain drives?
Eric/Optibelt: Waiting until the end of the design process before considering the belt drive is one of the biggest mistakes designers make. Particularly in the case of fiction belt drives, there is never enough room to allow for sound application practices in new designs. There is also a tendency to ignore things like minimum bending radius when trying to limit machine size and cost. With synchronous belt drives, the biggest mistake is to use service factors that apply to friction drives. Because timing belts don't slip, they are typically installed without accommodations for over load and clutching. This requires a much higher service factor to carry the load under severe starts and stops and to accommodate any built up inertia in the machine.
C.J./Drives Inc.: One error that designers often make in regard to roller chain is assuming that chain length is nominal. Roller chain is a repeated assembly of five primary parts, four of which - pins, bushings, pin link plates, and rollerlink plates - affect chain length. When designing a chain drive to carry or convey a product using attachments, the spacing between the individual attachments may be planned based upon the chain's nominal length. Exact distances between attachments, however, depend on the stack-up of tolerances of the individual component parts.
ANSI (American National Standards Institute) stipulates that roller chains are produced to nominal or above nominal length. For example, ANSI #80 chain has a pitch of 1 in., so 12 pitches of the chain should equal 12 in. Based on manufacturing tolerances and variations of the constituent parts, ANSI (ASME B29.100) allows the chain length to be a maximum of 12.016 in.
Designers also tend to overlook elongation stemming from wear. Wear is normal, should be expected, and actually can be used to estimate or approximate the usable life of the chain.
Roller chain is designed to be replaced when the combined wear of the pins and bushings equals approximately 3%. This wear/elongation naturally and correctly causes the overall length of the chain to increase. Many designers do not incorporate sufficient adjustment in the drive design and as a result, some usable life may be lost by the need to replace the chain before the maximum allowable wear limit is reached.
Dan/Gates: Chain: Under-design for actual load conditions: Very often, to reduce costs or use what's readily available, designers will build a product with a chain drive that's too small or has too little capacity. Under-designing a drive will reduce the performance of the product.
Use sub-minimal sprocket sizes: To obtain higher speed ratios or a more compact drive size, designers will frequently use smaller diameter sprockets and/or sprockets with fewer teeth than are recommended by chain manufacturers. The net effect may be greatly reduced service life for the drive.
Failure to consider capacity reduction due to connecting links: Chain power ratings assume the chain to be supplied in endless form from the manufacturer. However, the two ends of general purpose boxed chain must be linked together, and the connecting link limits and reduces the true power rating of the chain by as much as 30%.
Apply at too high of an operating speed: Running a chain drive at too high a speed without proper lubrication greatly reduces the service life of the drive and increases component wear.
Failure to consider lubrication, maintenance, and cleanliness: Roller chain drives require constant lubrication and maintenance. Because of oil and contamination problems inherent to chain drives, they may not be the best choice for food-handling operations. The best drive system for food handling is a synchronous belt with stainless steel, non-corrosive sprockets. Synchronous belt drives require no lubrication, are clean running, are maintenance free, and once installed properly, do not require retensioning.
Belts: Over-designing synchronous belt drive systems: Selecting wider belts and belts with more horsepower capacity than is actually needed can result in poor belt performance, increased maintenance, and more frequent replacement. Bigger is not always better.
Failing to design in a simple means of applying belt pre-tension: Designers often don't plan for proper tensioning by not including idlers or not allowing for the center distance to be adjusted (non-movable motor or difficult access to the motor).
Intermixing non-compatible belts and sprockets (using non-parent hardware): Poor, compromised belt performance will result from mixing synchronous sprockets from one manufacturer with the belt of another. Likewise, light-duty V-belts will have reduced service life on heavy-duty industrial drives.
Applying a V-belt drive in a low-speed, high-torque application: Roller chain and synchronous belt drives, rather than V-belts, are recommended for low-speed, high-torque applications.
Using sub-minimal tensioning idlers: Designers sometimes specify smaller-than-recommended backside idlers. This often reduces the service life of the belt.
What are some of the MISTAKES end users make?
Eric/Optibelt: For a friction drive belt, the biggest mistake is improper tensioning during installation. Best practices dictate that the belt be re-tensioned after an initial run period of about two hours. This could double the life of the belt. In most cases, however, production pressures do not allow for re-adjustment. Common mistakes with synchronous belt drives include misaligning the belt and pulleys and failure to match the belt and pulley profiles. Running a belt on the wrong pulley causes excessive wear, friction, and noise, and a shorter belt life.
Dan/Gates Chain: Not replacing sprockets with the chain as a set: Chains and sprockets should be specified as a drive system.
Not keeping fresh clean lubricant available: Chain lubricants can become contaminated, and therefore, should be replaced according to the drive manufacturer's recommendations. Oil filters should also be changed periodically to reduce chain wear.
Replacing original chain with lower cost competitive chain: This practice is often done to reduce maintenance budgets, but in the end, will increase the frequency of parts replacement and costs.
Belts: Replacing original belts with reduced performance or non-compatible competitive belts: Belt drive system manufacturers cannot guarantee the performance of their products if components are intermixed with components from other manufacturers.
Not replacing worn sprockets or sheaves: Although sprockets and sheaves are made of metal, they do wear out, especially in abrasive environments.
Not providing adequate belt installation tension: The shafts and bearings of a drive system are designed to handle proper belt installation tension loads. Inadequate tension will cause the belts to slip and fail prematurely. Designers should use the force-deflection method when tensioning a belt, or an easy-to-use and more consistent electronic device, now available from most major belt suppliers.
Not re-tensioning V-belts: Although V-belts are relatively maintenance free, tension should be checked on a regular preventive maintenance basis.
Not aligning shafts / sprockets / sheaves: Anytime a new belt is installed or a motor is moved, the drive should be rechecked for proper alignment and the belts should be re-tensioned.
Mishandling belts (crimping): V-belts and synchronous belts should never be twisted or sharply bent. They should not be hung for storage, but rather left in the original factory packaging.
C.J./Drives Inc.: The single most common mistake that end users make when applying roller chain in an application is the absence or omission of sufficient lubrication.
Lubrication is essential to any bearing, whether rotating or linear. In roller chain, the pin and the bushing act as an open journal bearing: The pin rotates within the inside diameter of the bushing. This bearing is not sealed and therefore requires continuous lubrication replenishment in order to prevent metal-to-metal contact and galling. Once the two surfaces of the pin and bushing begin to wear against each other and material is removed, the rate of wear between the two will increase until the usable and allowable service life is reached. Most roller chain manufacturers publish recommendations regarding the type and frequency of lubrication a chain should receive.
Another common mistake is subjecting chain to temperatures above the recommended maximum, resulting in either premature failure or accelerated wear of the chain's components. Generally, carbon steel roller chains are capable of operating from 0 to 350°F. Below 0°F, the material becomes brittle and individual components can fail catastrophically. Above 350°F, the heat-treated and hardened components may be tempered, giving up some of their hardness. This can increase the wear rates of pins and bushings.
MEET THE EXPERTS
Eric W. Steele
Power Transmission Product Application Department
Jim Discavage, P.E.
Senior Applications Engineer
Rapid Response Products
A Division of Fenner Inc.
(800) 327-2288, ext. 8255