Reasons for Tracking
In this respect the bigger the oven gets, both in length and width, the more ambitious belt tracking becomes.
In addition, today’s tendency to increase the baking speed to values of >35 metre/min. adds another challenge to the belt tracking issue.
So, to spend some careful thoughts on how the belt tracking can be done to satisfaction, is of real priority for the oven manufacturer and operator.
I. Oven Layout & Temperature:
- the 3 principle oven parts of entrance / oven chambers / exit and their design
- operating temperatures
II. Oven Drive & Tensioning Equipment
- drive of the oven with its system of driving drum(s), turning drum and support rollers
- tensioning station and guiding equipment
III. Belt Design & Tolerances
A belt can be:
- solid, of open or closed mesh structure
- with or without crossbars
- heavy or light weight
- with welded or looped edges
- stainless or mild steel
A) Belt quality
In most tunnel ovens mesh belts made of mild steel quality, which comes along in different trading names, are used. But occasionally, depending on the baked product, stainless steel is needed. All mesh belts are made of cold drawn wires, if they have a diameter smaller than ~6 mm. The cold drawing process has considerable impacts on the mechanical properties of the wires:
- it makes them much harder by increasing the tensile strength
- the range between the minimum and maximum tensile strength gets very big
- for stainless steel wires the mechanical properties are considerably higher than for mild steel because of its alloy elements like chromium or nickel
- the mechanical properties can vary considerably from one section to another within one belt, since it is not produced from one wire coil only
The higher the mechanical properties of the mesh belt wires are, the more tensioning forces in the oven are needed as part of the tracking. And, the higher the belt tensioning, the higher the risk that the belt starts to vibrate during operation.
So it should be in the interest of the oven operator to get a homogeneous mesh belt with no big variations in the mechanical properties within the belt and with not too high tensile strength values.
A good belt producer should be able to:
- influence this and
- give information about these values for their belts.
B) Belt type & crossbar
When it comes to mesh belt, be aware that those with crossbars in their mesh structure handle much better tensioning forces and are consequently easier to track than those just consisting of spiral wires.
For the crossbars there are belt types with crimped wires, which should be given preferences compared to the design with plain wires. The crimped crossbar keeps the spiral wire in position when the belt is tensioned. So the tensioning forces are absorbed equally over the width which is good for the tracking.
C) Mesh structure
Of great importance for a good belt tracking is an equal and rectangular mesh structure, especially for rolled baking oven belts (a.k.a. Z-belts) which are made just of flat rolled spirals.
The regularity of the mesh structure has the biggest influence on the tracking characteristics of a belt.
D) You can control the regularity quite easily with just some checks on the stretched non-tensioned belt:
- Count the number of meshes of your belt with random checks at different places along the belt. => Does your belt have the same number of meshes everywhere?
- Do random dimensional checks of the mesh openings. => Did all these measurements show the same figures?
- How is the overall appearance of the mesh structure? => Does your belt show an equal regular pattern?
- Is the belt’s row of meshes straight or running in a bow or wave? (You can check this easily with a (self-made) angle metre) => Does the mesh row run rectangularly and straight to the belt edge?
A good mesh structure of the belt makes your life much easier, when your oven line is running. Don’t forget that under operational conditions this structure changes. So especially the mesh’s angularity should be controlled regularly and is an indication for the wear life of the belt.
It is the mesh structure that shows the main difference between belts from different suppliers. With a quality belt from a good supplier these changes are only minute, even after several years of operation.
G) Belt edges
Mechanical damages to the belt edges can influence the tracking negatively. So it is best to repair them immediately and to maybe even replace this damaged part. Belts with looped edges are less sensitive to damages by side touches then the ones with welded edges. On the other hand, they are thicker in the edge area compared to the rest of the belt and often damages by and to the transfer knives at the oven’s exit appear. Mesh belts with welded edges are regarded by many users as more stable and less flexible in their mesh structure, since crossbar and spiral wire are firmly connected by welding. Because of this some suppliers concentrate on producing welded edges belts
With the increase of the belt tensioning the tracking forces have to be increased, too. A too high tensioning might make tracking difficult if not impossible and has some other negative impacts:
- the belt might start to vibrate and product falls off
- the belt edges might bend up- or downward since in the edge area it cannot stretch to the same degree than in the belt centre
- high tensioning increases the permanent mechanical impact on the belt’s wire structure and leads to a shorter operational life time.
H) Belt connection
Here is your checklist for connecting:
- Do belt connections at the joining ends thoroughly and properly.
- Do the connection with through going spirals or crossbars over the whole width.
- Z-belts need 2 connection spirals, one from each side.
- Take care that there is no change in mesh’s angularity of the joining pieces.
- Especially for rolled baking oven belts take care that there is no width difference in the two belt ends joined. Such width “jumps” might appear and can be overcome by the use of some “adapter spirals”
Non-properly made belt connections are predetermined trouble points for tracking issues.