Gravity, or non-powered conveyor can be either wheel or roller conveyors, and can be utilized for many applications. Gravity conveyors can be used to convey items either on a level, manually assisted push line or on a pitched line.
A wheel conveyor is an assembly of wheels mounted on a common, round axle and supported by a frame.
A roller conveyor is an assembly of metal rollers on a round or hex axle and supported by a frame.
Most, FLAT, FIRM BOTTOM items can be handled on either wheel or roller conveyors. Wheel type is preferred because of its lighter weight, easier portability and greater liveliness. A wheel conveyor requires less pitch and is less expensive.
When conveyor weight is an important factor, consider aluminum conveyor sections. Roller conveyors must be used for recessed bottom containers and certain other items that will not roll on wheels. Roller conveyors will also handle bales under certain conditions and some objects with cleated bottoms. Most items with irregular shapes and surfaces should use a powered conveyor.
The maximum load which can be conveyed on any roller or wheel section is determined by the following factors:
– Capacity of individual wheels or rollers and axles.
-Capacity of the frame. It varies depending on how the frame is supported.
– Shock loading through operational use.
The maximum “live load” the conveyor will see or the load of the product that will be on the section at one time is a major factor in selecting the proper conveyor section. Conveyor loading can be determined by one of the following.
Note: For SHOCK loads, a higher capacity frame or supports on 5′-0″ centers should be considered. EXAMPLE: If a 2 1/2″ H side frame is normal then go up to a 3 1/2″ H x 10 gat. Frame Assembly.
The correct wheel pattern is determined by the smallest package conveyed. To provide proper stability there MUST be at least three axles under the length of the item, with a minimum of seven wheels supporting the object at all times. The load should have a smooth and firm conveyable surface.
A properly selected roller conveyor MUST provide at least three rollers under the shortest load at all times. Thus, to determine the maximum center distance between rollers, divide the length of your shortest package by 3.
For example: Assume the conveying length of a package is 24 inches. This dimension divided by 3 is 8 inches. Eight inch is not a standard roller center. If we consider 6 or 9 inches would be standard. Since 9 inches would not provide 3 rollers under the package, 6 inch centers would be our choice.
Another consideration is load rating of the rollers. Many products have rigid conveying surfaces and are usually not exactly flat. In order to compensate for this, it is recommended to figure that 2/3 of the rollers will be supporting the full weight of the package. Therefore, it is necessary to take the weight of the product and divide by 2/3 of the rollers supporting the product to determine the capacity required of each roller.
– Will (3) rollers support the load in our example? Lets assume the package weighs 400 lbs. and is uniformly distributed. We would have 4 rollers under the load. Each roller would be supporting 100 lbs.
– If we consider the 2/3 rule we have (4) rollers under the box, but only (3) rollers supporting the load at one time, (2/3 x 4 = 2.666) or (3) rollers. Thus the roller capacity now needs to be 134 lbs. per roller.
For 6 inch roller centers to work in the example the following must be met:
1. The conveying surface of the package must be near perfect:
– It is a not likely that we will find a perfect conveying surface. Most packages have surfaces with varying degrees of irregularities. Different load conditions will be imposed on various rollers, depending on the degree of irregularities.
2. The 24 inch package must be held.
– If the 24 inch dimensions is not held, or the conveying surfaces has slightly rounded corners, the entire load coudl be supported by (3) rollers.
3. The frame rail must be able to support the load.
– The published frame rail capacities denote loads the frame rail will support without excessive deflection.
4. When the load is equal to the full capacity of the rollers, the distribution of the load should be considered.
5. If the adjoining sections are not at precisely the same elevation, excessive loads could be imposed on certain rollers.
A minimum of three (3) rollers should be under a package at any one time.
If three (3) rollers will not support the load, either specify closer roller centers or roller with greater capacity. It is better to specify rollers with greater capacity.
Consider the following examples for roller spacing.
Minimum of 3 rollers under bottom surface is ideal, less than 3 rollers and the load is unstable and will not convey. More than 3 rollers are needed under flexible bottom surface.
Use caution when conveyable surface does not have a bottom to contact the rollers. Pallets with five boards is a great example of this.
Gravity Grades: For gravity to generate movement of a package, the force of gravity must be sufficient to overcome the resistance of the rollers or wheels involved. When a conveyor is declined so much that the parallel force of the conveyor is sufficient to overcome the the resistance of the rollers at rest, the package will start moving.
The following should be considered when determining gravity grades:
1. Less grade will be required if the package has a “firm” conveying surface than if it has a “soft” surface.
2. Roller and bearings will effect the grade required.
– Non-precision bearings require less grade than precision bearings.
-Grease packed or regreasable bearings are not normally used for graded applications.
-The heavier the roller, the more it takes to move, once in motion it will remain in motion longer.
3. Heavier products will require less grade to move. when handling a variety of sizes, the pitch should be set for the heaviest package to prevent excessive speed and product damage to smaller packages.
4. Additional grade may be required if the product must start from rest after sitting on the conveyor for a long period of time, such as overnight, to compensate for carton indentation. Please not that a grade sufficient enough to start a line moving from a standing start may create excessive speeds in subsequent packages. Some ways to control speed are:
– Retarding wheels mounted under the carrier rollers or mounted between wheels or rollers to make direct contact with the product.
– Retarding rollers.
– Air operated brakes or pulsating brakes mounted under the carrier rollers.
Notes: Product flow may be erratic on gravity conveyor due to many factors. If smooth consistent flow is required consider using powered conveyor.
A package stop should be used at the discharge end of a pitched gravity conveyor if the conveyor is not feeding additional equipment.
Material Flow and Conveyor Systems has conveyors for every application.