Any belt or chain drive can transmit motion or power, but it takes a solid metal belt to serve as a conveyor of material, sorter of parts and synchronizer while operating efficiently in harsh environments, temperature extremes and in environments both caustic or corrosive.
Metal belts have been used in commercial applications for more than 30 years. From their beginnings in the space program, the applications for solid metal belts range from small, lightweight units used in precision machinery, to large, steel conveyor belts found in chocolate and food processing, material processing, warehouses, and package sorting facilities.
Metal belts are fabricated from materials often considered inflexible, such as high-grade stainless steel or carbon steel. As such, the belts have unique properties unavailable with more conventional belt materials. Some of the benefits include:
- A high strength-to-weight ratio. Belts of high tensile strength alloys have low mass and inertia. This allows using more of the input horsepower to move product--not the production line--to increase efficiency and reduce operating expense.
- Dimensional stability. Metal belts remain accurate because they do not stretch. Particularly important in precision conveying applications, such as sorting houses, metal belts can easily indicate where product is on a conveyor.
- Easily cleaned. Solid metal belts are inert, non-absorbent and suitable for corrosive environments. Organic acids and detergents do not affect stainless steel belts. Nor do the sharp edges of scraping blades or knives damage them. They feature a long service life, with a sanitary coefficient similar to that of glass or enamel. They comply with the most stringent sanitary requirements.
- Extreme temperature operation. Solid metal belts perform well at 1,100o F in ovens, as well as at -50o F in freezers.
- High electrical and thermal conductivity. Special alloys provide desired heat transfer, electrical conductance or magnetic properties. Material can be maintained on the conveyor at a known temperature, improving the quality and consistency of the finished product.
Metal belts are often used as friction drive belts. Generally speaking, straight power transmission is more easily accomplished with traditional drives, such as chain or V-belts. However, the properties of solid metal belts can be applied in more critical applications.
Some casting applications use a plain solid metal belt--often carbon steel or stainless steel--as the forming base for material being cast. If the surface characteristics need to be modified, coatings such as urethane can be added, which increase the belt's coefficient of friction. Teflon( coatings in a wide range of harness and temperature ratings can increase the belt's release characteristics.
In the chocolate industry, metal belt conveyor lines commonly link mixers, refiners and conches. The belts do not absorb odors or tastes, and there is no risk of flavor cross-contamination.
Laminating applications use metal belts in two ways: with one belt as a base platen holding a single material, or with two belts positioned face-to-face and the laminate material between them. The electrostatic discharge properties of metal belts enhance their utility in laminating applications.
The metallizing industry uses the belts as masking mediums to create package windows, cutoff lines and other custom features of the metallized components of packaging films.
A magnetized carbon-steel belt removes steel chips from the sump in a machine tool. A scraping blade can then remove the chips from the belt surface.
Mail order facilities, media distributors and package distribution centers use automated sorting systems with coded information stored magnetically on a carbon steel belt, which triggers gates or pushers to remove a single item from the belt at a precise location.
Conveyor-accumulators exploit the smooth surface of a solid metal belt. Such systems depend on the strength of the solid belt for conveying, and the low coefficient of friction on the belt's surface allows it to slip easily underneath the product being conveyed and accumulated. This increases the productivity of packaging applications because product can continue being manufactured upstream, even if downstream equipment is off-line.
In addition to endless metal belts, a metal drive tape can be used for reciprocal motion, such as that seen in a robotic arm. Drive tapes usually feature fittings at either end so that they can be secured to the motor and arm assembly. Their low mass and high strength offer durability, quick acceleration and stopping, and near zero backlash.
Perforated metal belts
Perforations can enhance the performance of a metal belt. A range of perforations, from specifically sized holes at fixed locations along the belt, to random "open area" patterns, are common in product dryer systems. Virtually any shape can be cut into the metal belt surface, offering unique tooling nests, which can be co-located with vacuum pressure from beneath the belt to hold parts in place. Applications typically use small perforations to pass steam or liquid through the belt to the material being processed. The use of perforated belts in commercial baking plants reduces energy consumption, resulting in less maintenance, easier cleaning and improvement of total product economy.
A popular misconception is that a solid metal belt is too large, heavy or bulky to maintain easily. Several termination schemes, however, enable the easy use, installation and maintenance of metal belts.
By far, the strongest belt connector is a solid, high-energy beam weld. Most belts are manufactured as endless loops, with the weld in place before installation.
Should system construction prevent the use of an endless belt, field welding is an option. This is also the case if a segment of belt needs to be repaired or replaced. The belt manufacturer provides or recommends qualified repair technicians.
Alternatively, metal belts can fitted with a field termination kit, which relies on perforations and rivets. The strip is fed into the machine until the lead end mates with the tail end for final riveting.
A strong and useful termination option is a full-width hinge, referred to as a [I]piano hinge[I]. Half is welded to each end of the belt, and when threaded through the machine, inserting the hinge pin ties the belt ends together.
Install metal belts with the least amount of tension or pre-load needed to achieve a non-slip drive. This minimizes stress on the belt, pulley, shafts and bearings. For most applications, a pre-load stress of 2,000 to 5,000 psi is sufficient. Over tensioned belts may develop crossbow, creating a belt profile similar to that seen on a tape measure. Significant crossbow causes a system to perform poorly, and tension should be reduced if the condition is observed.
No metal belt is perfectly straight. When assembled, the finished lengths of the edges are different, a phenomenon called edge bow, or camber. The nature of elastomeric belts minimizes the effect of camber on the system, and crowned pulleys keep elastomeric belts centered. However, metal belts don't stretch; they tend to migrate toward one side of a pulley or the other. The system must accommodate belt camber and keep the belt centered by using one of several tracking mechanisms.
A metal belt should always be in contact with the entire face of a pulley, across 180 degrees of wrap. A crowned pulley reduces the contact area, raising the stress at the center of the belt, an undesirable condition that should be rectified using other methods of tracking. Devices to adjust the pulley axis, such as pillow blocks, jackscrews or air cylinders, are ideal for moving one side of a driven pulley. This slight change in side tension moves the belt across the face of the pulley until it reaches a desired position.
Alternatively, a steerable pulley can keep the belt tracking properly. It has a steering collar and bearing assembly press-fitted into the pulley body, which is mounted, in turn, on a static shaft. Turning either the shaft or the steering collar changes the angle between the shaft and the pulley face. This allows making tracking modifications while the system is operating. The steerable pulley may be matched with optical sensors and servomotors to automate the adjustment process.
Flanges may also be used with metal belts to aid in tracking. Flanges should be made of a lubricious but strong material, such as glass-filled Teflon or Delrin(r) acetal resin.
Common V-belts can be assembled to the inner diameter of a steel belt, which will allow the V-belt to mate with a grooved pulley. This arrangement provides stable tracking, with the steel belt acting as the tensile member for the system.
Basic design guidelines
Guidelines for designing with metal belts include a few rules:
- Avoid multiple reverse bends. Each flex and straightening of a belt adds stress. Fewer bends results in longer belt life.
- Use friction drive, rather than a sprocket drive. This, too, simplifies belt design and improves belt life.
- Minimize the number of pulleys and sheaves because each affects tracking. Slides and supports may be a better alternative on long conveyor runs.
- Use the largest possible pulley diameter to reduce bending stress and increase belt life.
- Adjust the idler (driven) pulley shaft for easier belt alignment.
- Use the minimum initial belt tension to minimize stress on system components.
Solid metal belts perform a variety of services in today's complex manufacturing environment. Building on more than 30 years of application experience and constant advances in technology, new systems will continue to be developed for worldwide applications.