Debonding of Ceramic Lagging
Ceramic lagging has traditionally been applied to pulleys using two-part neoprene contact cements in a process generally known as ‘cold bonding’. Although this is a convenient system there are a number of limitations that can adversely impact the reliability and service life of the ceramic lagging, including:
- Bond strength between the rubber backing on the ceramic lagging and the steel pulley shell is limited and it does not achieve a 100% rubber tear bond. The generally accepted minimum bond strength for lagging adhesion in the conveyor industry is 9 kN/m – this is well below the level required for a 100% rubber tear bond. 100% rubber tear is where the bond strength is greater than the rubber, causing the test samples to fail in the rubber and leaving rubber adhered to the substrate.
- In a drive application with high belt tensions, where the shear forces applied by the pulley to the conveyor belt exceed the bond strength between the ceramic lagging and the pulley shell, the lagging can be ripped off the pulley shell (see attached photos).
Graph#1 illustrates how increased belt tension at the drive pulley increases the required lagging bond strengths, which can be well above the adhesion value of 9 kN/m, currently used as a requirement for cold bonded ceramic lagging.
Other considerations when using cold bonded lagging systems include:
- Bond strength can be adversely affected by moisture during application. High humidity during application of cold bonding adhesives can reduce the final bond strength by as much as 50%.
- The application of ceramic lagging using cold bonding adhesives relies on the skill of the operator and the method of application being used.
- Ceramic lagging applied with cold bonding adhesives has multiple joins between the lagging strips. These joins are a weak point that often allows water to pass through to the pulley shell and cause shell corrosion. In the worst case scenario, this can lead to failure of the bond from the edge of the lagging strip, with the resultant lagging failure. For pulleys that are being refurbished, the shell corrosion may require machining of the shell with the resulting loss of shell thickness requiring a de-rating of the pulley service load or, in the worst case, scrapping of the pulley.
Improved Lagging To Pulley Adhesion
1.) Hot Vulcanised Ceramic Lagging (HVCL)
Hot vulcanised rubber lagging is generally recognised as providing a number of advantages over pre-cured rubber sheets that are cold bonded to the steel pulley. The main advantage is the bond strength between the rubber and the pulley shell is significantly higher for hot vulcanised lagging. The second, often overlooked, advantage is that there is no join in the lagging when the rubber has been hot vulcanised. Using cold bonded lagging these joins can provide a starting point for shell corrosion, allowing the rubber to lift and be torn from the pulley.
A process for installing ceramic lagging using hot vulcanising has been developed . This solves any issues with lagging to shell bonding, providing a stronger and more reliable connection between not only the lagging and the pulley, but also, importantly, between the individual lagging strips. HVCL ensures a 100% rubber tear bond between the lagging and the pulley shell, resulting in considerably higher adhesion strength (refer to Table#3).
TABLE#3 – ADHESION TESTING HOT VULCANISED BONDED RUBBER/MILD STEEL
|RUBBER COMPOUND||TENSILE STRENGTH (MPa)||BONDING SYSTEM||ADHESION 90 DEGREE PEEL (kN/m)|
|76511||15.0 min||COLD||5.0 – 15.0|
The hot vulcanised ceramic lagging is the subject of a current patent application.
Removal of joins
An additional significant advantage of HVCL is that there are no joins between the ceramic lagging strips (see image#5). When HVCL is vulcanised the un-cured rubber at the edges of the lagging flows together and vulcanises to form a continuous layer that is strong and also impervious to water, preventing shell corrosion. This removes the potential weak points where the joins meet, which can lead to failure of the ceramic lagging.
Application of HVCL
HVCL is installed using a tightly controlled high pressure autoclave process, at elevated temperature and with skilled operators working in a factory environment, ensuring a reliable and consistent result.
2.) Direct Bond Ceramic Lagging (DBCL)
Direct Bond Ceramic Lagging has been used successfully in a large number of mining applications. DBCL involves glueing aluminium oxide tiles directly to the steel pulley shell with specially formulated epoxy adhesives. The key to this success of DBCL is the very high adhesion strengths obtained between the epoxy adhesive and the steel pulley shell and also between the epoxy and the aluminium oxide tiles.
These bond strengths are typically >20 MPa and 3-4 times greated than can be achieved with rubber.
DBCL has been used on both Drive and Non Drive pulleys. For Drive pulley applications care should be taken with regard to the potential problem of belt cover wear and the use of the “Lagging Analyst” programme (Overland Conveyor) to check for the this is recommended.