Many thousands of spiral conveyors serve the global food processing industry in applications ranging from cooling, proofing and freezing in the bakery sector; to freezing in the fish, poultry and red meat sectors; to pasteurizing and conditioning applications for manufacturers of sauces and cans. The reasons for this are clear: by comparison with standard conveyor systems, spirals minimize machine footprint, eliminate multiple transfers, maintain product orientation, enable a controlled environment for consistent product quality, lower energy costs, reduce manual handling, and increase product throughput.
As a result, our specialist spiral team has a full roster of new-builds and retrofits to complete. But we’re also frequently approached when food manufacturers experience problems with spiral belt surging (also called slip-sticking), when the belt fails to run smoothly; or when the belt is too tight and lifts on the outside (known as the Xmas-tree effect). Belt snapping and product movement are also challenges that have food processors requesting our support.
This new series of blogs explains the mechanics of low-tension spirals and how they work, and offers guidance on how to ensure optimized performance and years of efficient and reliable production.
My first blog article concentrates on overdrive: what it is, and how to manage it for maximum belt life and top operational performance.
Friction is key
Low tension spirals consist of a helix curve. They use the friction between the moving drum (or cage) and the inside belt edge to provide the drive force. The main drive has to overcome the friction force between the loaded belt and the support rails. Drive efficiency depends on the coefficient of friction between the belt edge and drum, and the belt underside and support rails.
Overdrive is positive slippage
The key relationship is between the speed of the belt and the surface speed of the drum acting on the inside edge of the belt. The drum surface must move slightly faster than the inside edge of the belt, giving positive slippage, and this is referred to as overdrive.
Overdrive is required in order to develop a frictional driving force for the belt. With no overdrive, or worse, negative slippage, the drum would act as a brake, severely damaging the belt.
The proper amount of overdrive is not a precise value; it depends on friction factors between the belt and drum, belt underside and support rails, and product load. A reduction in friction between the belt underside and support rails greatly improves operational performance and helps to maintain an optimum overdrive setting. But how to achieve this? Experimentation will be required – as all systems vary in some way. The first step is to measure the overdrive.
- Stucture assembled with
– columns and
- Drum or cage
- Cage bars
- Cage bar caps
- Drum drive
(or cage drive or primary drive)
- Radius belt
- Aux. Belt drive, (take up drive)
- Return path
- Take up and take up tower
- Dance roller
- Carry way
How to measure overdrive on a low-tension spiral conveyor system
- Stand by a static element (e.g. a structural column) as your starting location, place an object on the belt directly in line with an identifiable cage bar or point on the drum (make a mark on the cage bar or drum if necessary) and observe the system for one revolution back to your starting location.
- The cage bar will return to the starting location ahead of the object; the distance that it is ahead is the amount of overdrive.
- As a starting point, it is advisable to set the slip to a multiple of 3 to 4 vertical spacings (tier pitches).
- If the object was placed on the outside edge of the belt, allow a multiple of 4 to 5 tier spacings of overdrive. The optimal overdrive is usually specified by the manufacturer or belt supplier.
Be aware that each system has a different slip that depends on many factors. For example, ambient temperature and changes in the ingredients can all affect the slip. If the results are outside the parameters above, you may need to adjust your overdrive controls
Why it’s vital to check overdrive regularly
Overdrive should be checked regularly. When it’s sub-optimal, it can lead to a variety of problems, as set out below.
Excess or high overdrive
- Uneven belt operation (surging) because the belt becomes so loose that it loses contact with the drum and briefly stops until the take-up drive pulls it tight enough against the drum to drive again.
- This surging may cause problems with the product, due to excess movement.
- Wear of inner edge module, that is in contact to the drum or cage bars, is increased.
- Higher belt tension, which causes the belt to tend to flip up, and results in a shorter belt life.
The optimal amount of overdrive is therefore a compromise between smooth operation and low tension. As indicated above, each system is different and requires special treatment. For example, when using ribbed UHMW cage bar caps, less overdrive is required as the rib profile provides more drive force from the drum. Less overdrive is also needed when using a solid steel drum, as the driving force from the drum is increased due to a higher coefficient of friction between the belt edge and the drum.
The importance of cleaning
As explained above, friction is key, so thorough cleaning of the belt, drum and spiral installation is extremely important. Indeed, the main factor that influences the slip and performance of the spiral is the level of contamination that affects friction.
Sometimes the belt and installation seem to be clean to the naked eye, but in reality there can still be a film of fat or oil on the drum, and this will affect the friction. In my next blog article I will explain the importance of cleaning and give tips on how to clean your spiral installation in the most efficient and effective way.
Complexity calls for expert input
In our experience, many problems with spirals can be fixed by adjusting the overdrive settings. Our highly experienced spiral engineers are glad to assist customers who lack the confidence to do this themselves. Beyond this, Habasit offers full technical support services to optimize spiral operation, from fixing running issues, to replacing belts, to advising on retrofits and new constructions.