Horsepower isn't the only consideration
November 11, 2008
Setting up a roll forming line requires careful consideration of the speed reducers to be installed at each forming station. After determining the horsepower for each station, it is necessary to determine the appropriate durability, thermal horsepower rating, and strength for each speed reducer.
Because roll forming can be an aggressive, demanding application, improper use of speed reducers in a roll forming line can be costly. Problems can arise from reducer overheating, premature gear failure (excessive wear or tooth breakage), or installation misalignment. Just as important as speed reducer selection is gear type selection. In roll forming, spike loading is not uncommon, and in many cases, the material gauge and size of the shape being formed cause the load and speed to vary. It is necessary to choose a durable gearing system that can withstand shock loads and heavy loading at low speeds.
The first step is understanding the overall system configuration, which includes the number of stations, total horsepower input, the input and output speeds, and whether a transmission is going to connect the motor and reducers.
Whether the line is producing tube, pipe, corrugated shapes, or some other metal form, the forming process is similar. Either the motor drives the reducers directly or it drives a transmission that drives the reducers. The output shaft of the reducer is coupled to rolls that form the product.
The three basic considerations in selecting a speed reducer are:
After the number of roll stands and the overall horsepower required are ascertained, the next step is to determine the amount of horsepower applied to each reducer. For this example, consider a hypothetical tube mill that:
Because reducer ratings vary with speed, it is necessary to evaluate the reducer at both speeds.
Durability. Speed reducer durability is measured by service factor. Reducer manufacturers provide service factor tables based on the number of hours of operation and loading type (see Figure 1). Roll forming is considered to be a moderate shock application. Assuming that the line operates 10 hours per day, the minimum recommended service factor is 1.3.
Hours per Day
Because the total input is 100 HP and this line has 10 forming stands, the input to each reducer is 10 HP. Multiplying the required horsepower by the service factor results in the minimum mechanical horsepower rating of the reducer, which is 10 HP x 1.3, or 13 HP.
Thermal Horsepower. The next step is to determine thermal horsepower. The input horsepower for each reducer is 10, so each reducer's thermal horsepower rating must be at least 10. The reducer's thermal horsepower rating appears on the reducer's rating table (seeFigure 2).
Thermal HP With Fan
Shaft Strength. Two motor/trans-mission arrangements are possible. In one arrangement, the motor/transmission connects to a single reducer, and it drives all the other reducers. This puts a significant amount of stress on the first reducer because it reacts to all of the torque developed by the motor/transmission (see Figure 3). If the shaft cannot with-stand this much torque, it is necessary to redesign the line. The other arrangement places the motor/transmission in the middle of the roll forming line. This etup divides the torque between two reducers, halving the stress on each one (see Figure 3).
A few other considerations are important in designing a roll forming line.