by Daniel Pierre III, JN Machinery
Part 3: The Conveyor Mechanism
“A turned-up belt can’t fit the pathway of a flat belt. Converting to turned-up belt requires an entirely new conveyor frame.”
In the previous two articles in this series on conveyor ovens, we have focused on the heating system and the insulated box. This issue’s article takes a look at the most important and perhaps the least understood part of a conveyor oven, the conveyor mechanism. From a mechanical point of view, ovens are composed of simple, basic parts that may seem like commodity items. In reality, the quality of each component and the skill in which all the parts are put together is what sets one oven maker apart from another.
The conveyor mechanism of an oven directly takes the heat, bears the weight of the springs and goes through repeated temperature changes from room temperature to 950°F (or more). For this rea son, the whole mechanism must be allowed to expand and contract with heat. All moving parts must be able to move freely under all conditions, and all rigid or stationary parts must remain tightly in place.
Almost all conveyor mechanisms have a frame, which usually forms the sides of the heat chamber and is referred to as the “side rails.” Frames are made of steel and painted with high-temperature paint for applications less than l,000°F. For high-temperature applications, the conveyor frame needs to be stainless steel.
Drums/sprockets and shafts will be attached to each end of the frame with bearings. The drum or sprocket size and material will vary according to the belt specifications. Depending on design, some ovens require additional drums and shafts to support the return path of the belt. In all cases, the bearings have to be high temperature quality to withstand heat and loads. A good oven maker will use bearings with special clearance in them to handle the high heat. Make sure your maintenance programs include a regular high-temperature greasing schedule of these bearings. A good quality oven will have the grease fittings in an easily accessible location to make this task easier. Other non-lubricating bearings, such as those in back-up shafts, should be made with materials that can bear loads and high temperatures. Bearings do wear out over time, and when they start to wear, the deterioration rate is quite fast. A completely failed bearing can cause other components to wear or break.
All conveyor mechanisms have a motor to drive a shaft, which then drives a conveyor belt. Motors can directly drive a conveyor, or it can operate indirectly using a chain-and-sprocket system. There is nothing wrong with driving a belt indirectly, but it does add to the number of parts that can break or wear down. Another advantage of a direct-drive motor is the underside of the conveyor can be left clear to allow for large collection containers. There is, however, a big difference between DC and AC motors.
A direct current (DC) motor has a very simple speed control compared with an alternating cur rent (AC) motor. However, the DC motor gearbox usually has hard composite gears vs. metal gears in an AC motor. As a result, DC motors carry lower torque and do not last as long as AC motors. An other disadvantage of DC motors is that they have brushes that require maintenance and cause excessive motor noise.
A small AC motor with hypoid/ helical gearing is up to 80 % more efficient than worm gears found in a DC motor. High quality AC motors can also have a Hall I/C pulse generator built into the motor, which allows for more sophisticated belt speed control. (See more on this in the next article).
The most obvious part of the conveyor mechanism to consider is the belt itself. There are many types of belts available, and most oven makers supply different belts with their models. It doesn’t mean that any particular model of oven can accept all belt types, though, as I will explain later.
The four belt styles most spring makers will come across are: flat-edge belts, turned-up edge belts, solid trays, and flat- wire belts. These four styles have unique advantages of their own, but each application will dictate the belt style needed.
A flat-edge belt is a basic, economical belt. Since the sides of the heat chamber will always be wider than the belt to allow belt expansion due to heat, the gap on each side of a flat belt can cause problems. Over time, the cross bars of these belts can start to distort, causing the middle section to rise upwards, widening the gap at the belt edge. When this happens, springs tend to roll to the outside, and can get caught or slip over the belt edge.
There is an easy solution to this problem: Simply take the belt apart and flip it over and use the backside of the belt. In a very short while, the crossbars will straighten and the belt edges will return to flat. Flat belts should be flipped over once a year or when distortion is detected. With proper maintenance, a flat belt can last an extremely long time. In some cases, a side seal (essentially a very long extension spring) can cover the gap at the edges. However, some parts can get caught in the side seal. If this happens, your application may require a turned-up edge belt. These belts have either a 90° bend at each edge, or fins or flights attached at the edges. In both cases, there will be a solid edge that protects springs from getting caught between the belt edge and side rail. A disadvantage of a turned-up edge belt is these belts will also distort over time, but you can’t flip them over like you can a flat belt.
A question asked to oven makers every day is, “Can you convert a flat-belt oven to a turned-up belt oven? The answer is, “Yes, but it is not as simple as swapping in a turned-up belt.” A flat belt can be quite flexible, so it can wrap around small-diameter drums. A turned-up belt has limited flexibility because the raised edge cannot bend sharply. Consequently, a turned-up edge belt requires a larger drum than a flat belt does. The bottom line is that a turned-up belt can’t fit the pathway of a flat belt. Converting to a turned-up belt requires an entirely new conveyor frame. Whether you are using a flat belt or a turnedup belt, the secret for long belt life is to have mini mum take-up tension on the belt. Tension should only be enough to drive the belt with maximum spring weight on the belt.
The take-up tensioners are designed to help maintain enough tension so the belts won’t slip. They also function as a tracking device for the belt. As the belts heat up, they will stretch, and they may not stretch evenly from left to right. As a result, belts tend to wander over to one side. (This is especially true of new belts). A common mistake is to over-tension the belts or to try to correct the tracking quickly by turning just one side with several turns of the take-up screw. The best approach, however, is to turn a take-up screw just one-quarter turn and let the belt make its own correction. It is the same concept as making slight steering corrections when driving a car. If you over-correct your steering wheel, you will have to quickly make another correction in the opposite direction.
Mesh belts—both turned-up edge and flat-edge styles—can cause problems with small-diameter wire parts, especially torsion springs with legs that can become trapped in the belt mesh. There are two options to correct this situation: Option one is to coat the mesh belt with a silicon compound (RTV- 732). This compound fills in the voids of the mesh without affecting the flexibility of the belt. The silicon compound can easily be applied with a putty knife but does have a temperature limit of 600°F. Another option for this application is to use a tray belt. A tray belt is a solid surface that doesn’t allow any part of a spring to get caught or distorted while traveling through an oven.
The fourth type of belt often seen on conveyor ovens is the flat wire belt. It has extremely large openings—usually ½” x ½” or larger—and is mainly for large parts. An advantage to these belts is their relatively low mass lets the belt heat up quickly so more of the energy can be used to do actual heat-treatment of the springs. Another major advantage is that the belt is sprocket-driven, so take-up tension and tracking is not a problem.
One final thing to consider is that, since the conveyor mechanism is such an important part of an oven and it really is a self-contained unit, it can be possible to swap out an old, broken-down conveyor for a new one, resulting in virtually a new conveyor oven at about half the cost of buying a completely new oven.
Four Important Tips for Conveyor Mechanism Maintenance
- Make sure your bearings are high-temperature quality and grease them regulalry with high-temperature grease
- Flip your flat belts on a regular schedule or as soon as you see a distortion
- Minimize tension on the belts so they do not stretch prematurely and cause tracking headaches
- When changing tension on a belt, tighten only one side and only a little bit at a time
Figure 1: Conveyor mechanism ready to be installed in an oven.
Figure 2: Drum and shaft.
Figure 3: Flat-edge belt.
Figure 4: Side seal.
Figure 5: belt with side seal (top) and without the side seal (bottom).
Figure 6: Turned-up edge belt.
Figure 7: Take-up and bearing.
Figure 8: Belt coated with silicon compound.