This article describes the wide range of materials available to make rubber rollers, and provides a list of the many questions to consider in selecting a roll covering. To ensure that a broad range of design parameters are considered, it suggests six key areas of inquiry:
- Current Roller Specifications
- Mechanism for Roll Failure
- Mechanical Operating Conditions
- Environmental Operating Conditions
- Chemical Conditions
- Realistic Objectives
The choice of rubber (elastomer) materials for rubber roller applications is more challenging than choosing rubber materials for static applications. There are many dynamic conditions which affect the performance and longevity of the rubber compound. Some of the operating conditions present distinct challenges to the engineer since choosing a rubber material selected to maximize a physical property (release characteristic for example) minimizes another much needed property (the bonding of the rubber to the metal core can become nearly impossible with high levels of release agent chemistry).
Developments in recent years of new rubber compounds, elastomeric alloys, high performance fillers and polyurethane technologies have made the engineer’s job more difficult in selecting the best material for an application.
In years past, the rubber compounds available to choose from were the old tried and true compounds of natural rubber, nitrile (Buna-N), SBR (Buna-S), Neoprene, silicone, Hypalon and EPDM. Some of the formulas for these materials date back to the 1930’s and 1940’s. The advent of new high performance fillers, vulcanizing (curing agents), plasticizers and processing additives have given new life to some of these older materials. An increase in wear resistance, durability and performance can be achieved if the supplier has the capability to innovate, design and test their materials.
Design Criteria & Operating Conditions
This section provides a list of the many questions to consider in selecting a roll covering.
1. Review the current roll specifications including:
- Roll diameter
- Maximum and minimum diameters
- Cover thickness
- Maximum and minimum
- Hardness specification
- Shore A (durometer)
- P&J (plastometer)
- Edge relief (end dub)
- Surface finish process critical?
- Specified numerically?
- Ra, Rz, Rt, other
- Cut off length (filtering length, 0.03 std)
- Specified numerically?
- Total runout (TIR, concentricity, etc.)
- Crown amount and shape
- Profile (taper, straightness, etc.)
- Bearing journal influence
2. Understand the mechanism for roll failure or cause for removal from service.
- When does this first appear?
- At what point will it render the roll unsuitable for service?
- What condition is the roll cover in when removed from service?
- If the roll is crowned, how much crown is remaining?
- Are there cracks in the cover?
- Direction of cracks
- Estimated depth
- Location of cracks
- Is there a change in hardness before regrinding?
- Can the roll be reground and placed back in service?
- How much minimum material is removed each time it is reground?
- Will a reground roll operate as long between grinds as a new cover?
- Is there a change in hardness after grinding?
- Are nip impressions taken and analyzed?
3. Review the mechanical operating conditions including:
- Is the roll nipped?
- Number of nips
- Operating speed
- Surface fpm or rpm
- Is the roll driven?
- Surface speed differential between nipped rolls?
- Pressures (loading-PLI) both existing and future plans
- Deflection of roll
- Operating speeds
- Continuous or intermittent and future speed upgrade consideration
- Nip widths (uniformity)
- Actual nip pressure (psi)
- Nip dwell time (residence time in milliseconds)
- Web tension
- Wrap angle
- Balancing considerations
- Dynamic (ISO 1940/1 grade level?)
- Static (static unbalance tolerance on roll periphery)
- Crown shape
- Cosine (what degree?)
- Simple (tapered)
- Is a release characteristic important?
- Surface finish
- Static electricity potential
- Does cover need to be non-marking?
4. Review the environmental operating conditions including:
- Temperature (continuous duty, intermittent)
- Source of thermal input (rolls, web or other)
- Steam heated (rotary joint)
- Hot oil heated
- Induction heated
- Recirculated chilled water
- Resistance heated
- Direct heat
- Indirect, etc.
5. Review the chemical conditions and other considerations:
- Process chemicals
- pH of chemicals
- Chemical contact
- Cleaning chemicals
- Wiped only
- Does roll have opportunity to “dry off’ from cleaning chemicals
- FDA compliant materials needed?
- Is hydrolysis a factor to be considered (affects mostly urethane)?
6. Define a realistic objective in redesigning the roll covering material
- Inputs from machine operators, maintenance, engineering
- Can any adverse operating conditions be changed to affect this objective?
- Changes in speeds, pressures, temperatures, chemicals, etc.
- Add internal cooling of roll as necessary
- Will changing any roll parameter help achieve the objective?
- Changing diameter, crown, dub, cover thickness, runout, etc.
- Is there an existing material in process to accommodate these conditions?
- If not can an existing material be modified?
- Does a new material need to be developed?
- Is the material cost effective for this application?
Source: Imperial rollers.
The above content is being compiled for educational purpose and printers can use this for reference purpose.
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