belt drive technology - design and usage guideline

Unlike many old machines that employs gears for transmission method, Newer machines began to grow fond using belt drive, since it's cheaper, offers safety (belts will break on overload, unlike gears that take everything down with it) , easier to apply and it's easier for maintenance. One thing that we might forgot to highlight is that the belt drive efficiency actually depends a lot in heat factor.

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). Even though 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.

Here are some points designers need to know :

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.

Here are some points end users need to know :

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.

And here are some shortcomings in design of chain and belt drive that might reduce it's performance:

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.

Some mistakes to be avoided by designers :

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.

Wrong service factor applied. 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.

Using sub-minimal tensioning idlers: Designers sometimes specify smaller-than-recommended backside idlers. This often reduces the service life of the belt.

Some mistakes to be avoided by end-users:

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.

Whenever applying transmission system on a machine, check it's function and compatibility, transmission system provides better result only when used in a proper environment