Selecting speed reducers for roll forming lines - Horsepower isn't the only consideration

By Mitch Machelski

October 14, 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.

Three Selection Considerations

The three basic considerations in selecting a speed reducer are:

1. Durability.
2. Thermal horsepower rating, or its ability to dissipate the heat generated.
3. Strength of the initial reducer connected to the motor or transmission.

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:

• Has 10 roll stands (four breakdown passes, three forming passes, and three finishing passes).
• Uses a 100-HP motor that operates at 1,750 RPM.
• Has a two-speed transmission between the motor and reducers.
• Has gear ratios of 1-to-1 and 2-to-1, resulting in reducer input speeds of either 1,750 RPM or 875 RPM.

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.

Figure 1
To calculate a reducer application's service factor, find the intersection of the correct row (based on the number of hours of operation per day) and the shock-loading column that matches the application. In this example, the service factor is 1.3.

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).

Figure 2
A typical reducer rating table includes horsepower ratings, efficiency, and output torque. This particular reducer has a mechanical horsepower rating that exceeds 13 at 875 and 1,750 RPM, so in this characteristic it is sufficient for the hypothetical roll forming line. However, its thermal horsepower rating without fan is less than 10 at 875 RPM, so it requires a fan to dissipate the heat generated.

Shaft Strength. Two motor/transmission 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 (seeFigure 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 setup divides the torque between two reducers, halving the stress on each one (see Figure 3).

Other Considerations

A few other considerations are important in designing a roll forming line.

• Cooling
  • If a fan-cooled thermal rating isn't sufficient for the application, a cooling coil can increase the thermal rating. The cooling coil is placed in the oil sump of the reducer and water circulates through it, removing heat.
• Drive ratio
  • In some cases, one section of reducers must have output shafts that rotate at different speeds. An unequal-ratio drive can handle this application.
  • In some roll forming lines, as the material passes from one section to another, the speed changes, causing preceding drives to overhaul or back-drive. A back-drivable ratio may help reduce damage to the gearing.
• Gears
  • Three main worm gear characteristics make them suitable for roll forming: the ability to withstand heavy shock loads, compact size, and right-angle design.
  • Double-enveloping worm gearing, in which both the worm and gear envelop each other, allows more tooth contact than cylindrical worm gearing, increasing its capacity (seeFigure 4). However, cylindrical worm gearing is less expensive than double-enveloping worm gearing.
• Installation and mounting
  • Aligning the high-speed shaft is critical to prevent fatigue failures. Aligning the reducers from a common datum is critical.
  • Using flexible couplings to connect the high-speed shafts eliminates additional misalignment.