What are Process Rolls and Rollers?
Process rolls and rollers are any type of cylindrical rods used for processing different types of media. Roller functions include roll forming, rolling, embossing, shaping, bending, and stretching products and materials.
This video shows the bending of sheet metal through a roll bender machine. Video Credit: Gotmachinery
The selection process for industrial rollers can be very application-specific. For most applications, however, it is important to consider the overall performance specifications, the construction of each roller component, and the materials of construction. Additionally, industrial buyers should note the construction materials of the roller and any environmental factors which could influence operation.
Performance for process rolls and rollers can be measured based on load rating and precision. These specifications determine the success of
- Load rating or capacity is the maximum force a roller can support or generate. This includes weight from the roller tubing and bearings as well as force sustained from a load. There are a number of factors that can affect a roller’s load rating, including axle length, roller length, and bearing capacity.
- Precision is a qualitative measure of the uniformity of a roller’s products. In addition to applying tolerable load on the workpiece, precision is affected by roller alignment and surface quality. Defects on the face of a roller and improperly aligned equipment will cause deformities and inconsistencies in the workpiece, and can increase machine wear.
Regardless of the type or application, rollers are constructed from three distinct elements: tubing, bearings, and an axle. The tubing is the cylinder of the roller that makes contact with the workpiece or product and performs the operation. The axle connects the roller to the surrounding machine or system from which it operates. The bearings connect the tubing to the axle, allowing the tubing to rotate independent of the axle while still being supported by it.
Figure 1 – Diagram showing the three components of a roller. Image Credit: Mathews Conveyor
Rollers can be constructed in numerous ways depending on the applications. Facets of construction include types, methods, and specifications.
All roller applications require the cylinder to make physical contact with a workpiece or other material, making tubing construction crucial to roller selection. Tubing construction can be described mainly by its physical dimensions.
- Roll diameter is the outside diameter of the cylinder tubing which determines the size of the roll. Larger diameter rollers are needed for forming applications where larger cylinders are required to shape or coat a workpiece effectively. In printing or embossing, rollers of varying diameters each have individual positions and different tasks within the system.
- Gauge thickness is the thickness of the tubing wall. Larger thickness is important for roller integrity and performance in forming processes that require large amounts of force. Gauge thickness can be expressed as a gauge scale or in distance units.
Figure 2 – Diameter and gauge lengths of a roller.
- Runout is the uniformity of a roller’s diameter around its circumference. A roller with perfect runout would have no ovality or face deformities across its entire length. An imbalance in runout will cause greater workpiece imperfections and can even be dangerous when rotating rollers at high speeds. Roundness should be checked not only at acquisition but during application for stress-laden processes that could cause deformation of the roller.
Figure 3 – Depiction of roller runout. Image Credit: Micro-Epsilon.
- Clearance or lift is the distance of two rollers from each other or from the roller to the support surface. In the case of sensitive processes and high material accuracies, the lift of the upper roller must be checked constantly so that imminent rejection of a workpiece is avoided. For certain applications, lift can be adjusted for tasks involving multiple size workpieces.
Figure 4 – Two rollers forming a sheet workpiece, where the clearance is only a fraction the size of the roll diameter. Image Credit: Micro-Epsilon.
- Camber is the built in deflection present across the length of a roller for forming and high pressure applications. Camber is essential for uniform deformation across a workpiece, since the pressure applied by a straight roller is not evenly distributed.
Figure 5 – Comparison of cambered to straight cylinder shape.
- Face length is the distance measuring the span of the cylinder face. It determines the length portion of the surface area contacting the workpiece or load. Face length must be sized at minimum to accommodate the workpiece and at maximum to the span between the frames.
Bearings are an important part of roller system design because a number of roller properties including load rating and noise level are dependent on how they are constructed. (See Definition of Terms at bottom for an explanation of bearing terminology)
- Precision bearings have hardened and ground ball races, ball retainers, and steel seals. They are used when the required load or operating life exceeds that of comparable non-precision bearings. They also are better suited for higher speed applications, low noise levels, and environments which require rugged seals.
- Non-precision bearings have stamped and coined outer races with machined cone ball races. They do not use ball cages or retainers, and most seals are steel or felt. They are an economical alternative to precision bearings for applications that do not have as demanding speed or load requirements. They also are suited for applications requiring a low coefficient of rolling friction.
- Journal bearings are all-plastic bearings made from nylon or a nylon-polyoxymethylene combination. They do no incorporate balls or seals. They are used when the requirements for precision bearings, except for low noise level, do not exist. They also are suited for applications requiring frequent washing or that allow for higher corrosion potential, and where higher friction coefficients are permissible.
Axle length and roller length are the key specifications for axle construction.
- Axle length is simply the length of the axle component of the roller. The axle must be long enough to fit into the frame it will be mounted on, but short enough to handle the load capacity without exceeding the deflection tolerance.
- Roller length is the length of the entire roller from frame to frame. It is the length of the roller once the axle is inserted into the frame. To minimize axle deflection, the supporting frame should be as close as possible to the ball bearings. Roller length is an essential specification to provide to a manufacturer designing a custom roller, as it allows them to determine the proper tube length and bearing size and position.
Materials of Construction
Select rollers made of material(s) that exhibit the most essential properties (e.g. strength or hardness) for the application.
- Steel and steel alloys are metals for which iron is the primary constituent. Steel is the master alloy for many types of rolls that include other metals as modifiers or surface coatings. Steel is commonly alloyed with manganese, nickel, chromium, and molybdenum to enhance properties such as strength, corrosion resistance, and malleability. Carbon steel contains larger amounts of carbon which increases the material’s strength and hardness. Hardened steel refers to steels that have been heat treated through tempering and quenching in order to increase hardness and strength.
- Stainless steels are steel alloys where chromium is the primary modifying metal. Chromium enhances steel’s corrosion resistance, making stainless steel the optimum choice for working with corrosive materials or in corrosive environments that also require superior strength and durability.
- Aluminum alloys are used to manufacture rolls for applications requiring a corrosion resistant and/or low density (lightweight) material, yet moderate strength. Aluminum can be significantly hardened with the addition of copper, magnesium, manganese, silicon, and other alloys in varying amounts. Aluminum can be less expensive than stainless steel for corrosion resistant applications. Aluminum alloys can be anodized to increase corrosion and oxidation resistant properties as well as to increase surface hardness to resist scratching.
- Composites are favored in terms of their reduced weight and inertia. Composites include nylon, polypropylene, polyurethane, fiberglass, and carbon fiber. Softer composites are utilized for printing, gravure, embossing, and stretching applications, and some (such as fiberglass) are sold as replaceable roll sleeves. Carbon fiber can replace metal rollers for pressing or forming applications for a higher cost; its lower weight reduces drive requirements and will cause less wear on bearings.
- Rubber is used to create rolls used for stretching, coating, embossing, and other non-machining roller processes. Rubber is a soft and flexible material better equipped than metal to handle delicate or sensitive products. Some processes utilize rubber rolls with grooves for a specified grip or to imprint certain shapes on a workpiece.
- Teflon coating can be added to a metal roller to prevent material from sticking on its surface. These rolls frequently require recoating as the Teflon wears off with time from repeated surface contact.
Certain environments may require special bearing or roller materials for proper functioning. For example, high humidity conditions require proper greasing, dusty conditions requiring sealed bearings, and excessive heat conditions require dry or high temperature greased bearings.
It is also important to note that the load capacity of a roller system cannot exceed the load capacity of the frame that holds the roller. In order to prevent failure, the frame must be capable of supporting both the load of the roller itself and the force a roller supports or applies.
Definition of Terms
Ball bearings – Balls in non-precision bearings are made of hardened steel. In precision bearings they are made of hardened chrome alloy steel. Retainers in precision ball bearings provide ball separation, minimizing contact friction and allowing higher operating speeds.
Races – These are the inner and outer surfaces that balls ride on.
Seals and shields – Seals are used to keep foreign contaminants out of the bearings and to keep desired materials such as grease in the bearings. Shields are used to protect seal from damage or wear.