In the material handling industry conveyors are used to move material from one point to another. Regardless of whether you are an OEM designing a system or the end user who operating the equipment, each of us is concerned with productivity curves (available uptime) and the costs associated with operating a material handling system.

Many variables influence the design criteria when building a conveyor system.

The obvious factors include:

• motor horsepower required driving the system
• type of material (and density) to be conveyed
• length and width of conveyor belt
• and last but not least how much material will be moved per hour.

The less obvious factors include:

• cycle starts and stops per hour or day
• turns, inclines, declines, merging into other conveyors
• how the material is loaded or unloaded onto the conveyor (shock loading)
• type of conveyor belt (solid rubber, chain link, plastic link, just to name a few).

So how do we match a speed reducer to a given application?

First, gathering information from the OEM or end user is of primary importance. We know this can be a bit of a challenge as the information may not be readily available because the project is still in the design phase or it is a replacement for an existing installation in the field. But whatever the circumstances may be, gathering the correct information in the beginning will lead to a successful installation and a more satisfied customer.

Here are a few basic questions to get started:

1. What motor frame size, horsepower and RPM are required? Check out the motor name plate for this data.
2. What type reducer output connection (flange & solid shaft or hollow shaft) will be used to mate the reducer to the conveyor? Hollow shaft connections will allow the conveyor head shaft or pulley shaft to precisely fit inside the hollow shaft. These are typically used in keeping the reducer close to the conveyor when confined spaces or clearance issues are present. The advantage is obvious as you minimize space and there are no overhung load issues to contend with. The disadvantage is “depending upon the application environment”, fretting and corrosion tend to take its toll on the mating surfaces thus making it difficult to remove the reducer from the conveyor. Solid shaft connections will require a flange coupling or other device to interface the speed reducer to the conveyor. The advantage is it is very easy to exchange or service the reducer or conveyor when needed. The disadvantage is that it takes up more space and the overhung load factor must be considered.
3. How fast will the conveyor move (usually expressed in feet per minute (FPM)).
4. What is the environment the speed reducer will be working in? Examples could be washdown in the food industry, outdoor installation in direct sunlight for bulk material movement, inside a freezer for moving material in cold storage, and the list goes on and on…

Most conveyor OEMs can readily provide this information and will usually have factored in all the variables that influence the speed reducer selection. An end user, on the other hand, may not have all the details and are looking to swap out the reducer of an equal size due to previous failure modes or poor service response from the supplier. In either case, the rule of thumb is “there is no such thing as having too much information when specifying a speed reducer for a conveyor application.”

So now that you have a basic understanding of the questions to ask, let’s work through a typical material handling conveyor example.

Assume the following example was provided from a Conveyor OEM:

Worm Gear Speed Reducer Drive Package Required Operating at:
1750 RPM ± 3%.
1.5 HP (as indicated on the motor nameplate)
Conveyor FPM = 300 ± 3%
Conveyor Pulley Diameter = 3.5 inches
Requested application service factor (sf) = 1.25 over motor nameplate HP
Application is uniformly loaded with continuous duty cycle of 24 hours per day, 5 days per week. It will be an indoor installation, horizontally mounted, with free air movement around the motor and reducer. The reducer will not be serviced by maintenance and is expected to run 2 years with a “shaft to shaft” warranty.

Based on the information above, let’s conduct a first pass estimate to approximate the size of the reducer required including service factor.

1.5 Motor HP x 1.25sf = 1.875 (rounded up to 2.0) Reducer Horsepower. Use the Boston Gear catalog to see what size reducer you need in terms of center distance. Under this scenario, we select 721 center distance size.

How to Determine Conveyor Speeds

Using some simple math we can quickly determine which WGSR (worm gear speed reducer) will initially fill the request.

The Dreaded Math Test:
The basic formula to find the reducer ratio for a given conveyor belt speed (FPM) is expressed as:
Ie = (Ms ÷ FPM) x Pd x (p÷12)

Where as:
Ie = Reducer Ratio
Ms = Motor Speed in RPM
FPM = Conveyor belt speed in FPM (feet per minute)
Pd = Conveyor pulley or head shaft diameter
(p÷12) = 0.261799

Example:

Ms = 1750
FPM = 300
Pd = 3.5 inches

(1750 ÷ 300) x 3.5 x 0.261799 = 5.3450 ratio required based on the parameters given

As an additional check to verify your work, substitute or rearrange the formula above to solve for conveyor belt speed (FPM) if you know the reducer ratio. This is expressed as:
FPM = (Ms ÷ Ie) x Pd x (p÷12)

Example:

(1750 ÷ 5.3450:1 ratio) x 3.5 pulley diameter x 0.261799 = 300.00353 FPM

We look in our Boston Gear catalog and find a 721 with 2 HP that comes in a 5.0:1 ratio. (5.34/5.00 = 6.90% difference). This exceeds the ± 3% as defined by the customer so what do we do?

Option include:

A. If an inverter duty motor is in use, then an adjustment of the motor RPM by 6.9% would correct the situation. 1750 x (1- 6.9%) = 1629.25 RPM.
(1629 new motor speed ÷ 300 FPM) x 3.5 pulley diameter x 0.261799 = 4.975:1 ratio which is much closer to the catalog standard of 5:1 and will give the customer the 300 FPM. Of course, there would need to agreement regarding the motor speed change.

B. If the customer has a fixed motor speed but could change the pulley or head shaft diameter by 6.9%, this could correct the situation. 3.5 x (1-6.9%) = 3.2585 new pulley diameter. We also assume that we will use our standard ratio of 5.0:1
(1750 ÷ 5.0) x 3.25 new pulley diameter x 0.261799 = 297.796 FPM which is much closer to the original specifications. Of course, there would need to be agreement regarding the pulley diameter change.

C. A less conventional but very low cost alternative could be a chain sprocket and pulley configuration that would interface the speed reducer output shaft and the conveyor head shaft. This configuration would give the speeds requested, but space constraints could be a limiting factor. Under this scenario, the speed reducer would require the chain and sprocket plus an adjustable mounting plate to compensate for chain tension. Overhung loads would also need to be considered for proper shaft and bearing life. Boston Gear engineering would gladly get involved to work through this option.

D. Develop a special ratio meeting the exact requirements of the customer. While this is ultimately the best solution, sufficient lead time is required to design and manufacture the gear set. Delivery can vary depending on gear tooling availability.

The key take away from this article is that given the proper attention to data collection in the beginning of the process, there are many options at our disposal to assist in problem solving and achieving customer satisfaction.

We hope this article informative and useful. Feel free to contact any of the engineering staff at engineering@bostongear.com for additional information.