Regular planned maintenance is key to maximizing the working life of a bake oven belt.
This should include a formal schedule of checks, not only to the belt itself but also to every part of the overall conveying system, i.e.
- riveted or welded joint
- terminal drums
- belt support
- guide rollers
- safety scrapers
- rotary brushes and back-up rollers
- break points
- tension device
Following are some of the conditions that can be faced by engineers when checking a bake oven belt:
This can be caused by drums, idlers, skid bars or break points that are dirty, worn or incorrectly adjusted. Another cause could be uneven temperature across the width of the belt width. All can be resolved through cleaning, replacement or adjustments.
Side wandering can bring the belt into contact with the structure or another rigid object, resulting in burrs. The edges of the belt should be filed to a round finish.
Scratches on the underside
This would suggest worn or damaged belt supports or safety scrapers. These parts should be replaced and the belt may need lubricating.
Potential causes include side wandering, uneven pressure from the belt cleaner or scraper, or a belt scraper that is wider than the belt itself. After correcting the cause of the damage, the belt edge should be checked and, if damaged, trimmed then carefully filed so that it is round.
Wavy edges and/or small blisters
This is a sign of uneven temperature and any deformations usually disappear when belt temperature is uniform.
Deformation can also be caused by problems with drums, belt supports or other parts coming into contact with the belt. Remedial actions include cleaning the drums, cleaning or replacing belt supports, and checking scrapers, rollers and belt tensioning. If a belt shows concave or convex deformations, your belt supplier will be able to rectify this using a specialised levelling device.
Loose rivets in the joint
This can be caused by normal wear or incorrect belt cleaning; loose rivets must be drilled out and replaced.
Hairline cracks in welded joint
This will require specialist repair welding – contact your belt supplier.
Hairline cracks in belt
If a crack is found at the belt edge, a crescent-shaped piece may be cut out
and the edges filed so that
they are round. If the crack is within the belt, small holes
can be drilled at each end
of the crack to prevent it
from spreading. If the crack develops
from the edge of the belt
and is so long that the belt
edge cannot be cut, a small
hole should be drilled at the
end of the crack.
Areas that have suffered more serious damage will need to be cut out and a new piece spliced in. The length of the new section must not be less than 2/3 of the circumference of the drum.
Incorrect belt tension
Belt tension problems are usually a result of temperature variations or issues with the tension device. The specific tension stress should be a minimum of 10 N/mm2 (1450 psi) over the belt cross-sectional area. Check whether the tension is correct by measuring the sag of the belt between two idlers in the bottom strand. Check tension device, counterweight or tension springs.
If belt vibration and/or slight wear is observed, this often indicates an urgent need for lubrication. Ongoing lubrication in the form of graphite is an essential maintenance task for the following reasons:
- reduced friction
- longer belt life
- less wear on skid bars.
- maintains even belt colour (dark/light areas will result in an uneven bake)
- helps prevent belt distortion
Many users manage this requirement by installing skid bars made from graphite. If this is not possible then graphite should generally be applied to the inside of the belt once a month for the first three months, then every three to six months.
The period between applications can be relatively long on belts used for products with a high fat content as fat will spread to the underside of the belt, whereas those used for dry products require more frequent attention.
If graphite is not permitted for any reason, specially selected oils can be used instead. However, it’s important to be aware of the risk of rust on belts (e.g. at weekends when the oven is not working), especially when dry biscuits are baked in convection ovens.
Regular and effective cleaning is probably the most important maintenance task in relation to bake oven belts and on solid steel this is usually carried out using rotating brushes, with the application of scrapers as necessary. The brushes should be adjusted to apply gentle cleaning of the belt surface and the belt should be rubbed manually with clean cloths at the same time.
Belts used for baking particularly sticky products can also be wet washed if required, but it is essential that the belt is oiled or conditioned when left standing for any length of time.
Perforated bake oven belts require slightly different treatment, with brushes used to clean the inside and outside the belt. If wet cleaning is employed, the washing unit can be either open to drain or equipped with a sludge pump.
Ineffective cleaning will result in a build up of carbon deposits on the baking side of the belt, leading to less than perfect products and, potentially, a risk to human health due to the presence of hazardous acrylamide.
The removal of these deposits can be difficult and time-consuming, usually involving the application of chemicals, dry ice or detergents and great deal of manual input. Following is a brief summary of the methods traditionally used:
A solution of cornstarch and caustic soda is applied to a warm, slow moving belt using mops or brushes. The loosened carbon deposits can be removed with a hand scraper. Once clean, the belt is washed, rinsed, dried and conditioned.
This method involves shot blasting the slightly warm belt with dry ice to remove carbon deposits. Again it will need conditioning afterwards.
Mainly used on belts with sugar-based carbon deposits, this method uses detergent and nylon scrubbers and, again, post-cleaning conditioning is necessary.
Baking release agent
The release agent is spread in an even layer at the entrance to the oven, softening of the carbon as it passes through the hot oven. Scrapers remove the carbon and the belt cleaned with felt wipers on the bottom strand. It is not necessary to condition the belt after cleaning.
An alternative to these time-consuming processes is Sandvik’s QuickCleaner system. This blasts a combination of baking soda and calcium phosphate onto the belt at high pressure to remove the bulk of the residue. A chemical action then attacks the organic residues at a molecular level. Capable of reducing cleaning times by as much as 70%, the process is dry (no risk of rust), safe (no dangerous chemicals) and suitable for use on solid or perforated bake oven belts.