Overcoming Inertia

The toughest engineering problems often require overcoming multiple, interrelated obstacles. A combination of greater than expected load inertia, reducer failures and contamination is what created a challenge for engineers working to fix a troughed conveyor at the CEMEX Balcones Quarry.

To eliminate conveyor drive underperformance and failures, CEMEX ended up replacing two motors and gear reducers with a single drive unit on the conveyor that feeds a screen serving a secondary crusher at the quarry. But to solve the problem itself, they worked with Plant Fabricators Inc. of Floresville, TX and Rexnord Industries to develop a total package for upgrading performance.

“The biggest problem,” says Brett FitzSimon, a designer at PFI, “was that it was difficult to get the conveyor started under a full load. Load inertia proved to be greater than expected, due in part to a combination of wider than standard idler spacing and the use of Garland-style idlers on the tail section of the conveyor which swing forward under load at startup.”

Actual horsepower required for proper operation was probably closer to 400 than the 250 hp available through the dual drives. Plus, it was difficult to synchronize the two drives, since no two motors are perfectly alike. As a result, there was imbalance between the two sides that caused inefficiencies and accelerated wear.

The second major problem, according to FitzSimon, is that the internal backstops on the original reducers failed repeatedly, contaminating and destroying the reducers, as well. When the backstops engaged, the twin drives had difficulty overcoming the load to restart the belt.

Rather than simply replace the drives, PFI and CEMEX worked together to come up with an engineered solution. The two drives were replaced with a single 500 hp motor and a Falk Drive One right-angle speed reducer, also rated at 500 hp. The drive is direct-coupled to the head pulley with a hollow-shaft mounting. It incorporates carburized and ground gearing, which provides a higher rating in a more compact package.

To further protect against contamination, it also is equipped with an AirMax™ breather which incorporates special membranes and intake pathways that restrict moisture and contaminants from entering the gear drive. An external electric cooling fan on the reducer lowers operating temperatures and increases thermal capacity to utilize the drive's available mechanical horsepower rating.

The motor is coupled to the reducer with a Wrapflex® coupling, which minimizes misalignment potential and incorporates a tough, flexible polyurethane element that does not require lubrication. The coupling is protected by a rotating shaft guard.

To eliminate the problems caused by backstop failure, PFI used an external backstop that incorporates oversized, heavy-duty cylindrical rollers with a wide contact area and a rolling action that minimizes wear and increases reliability. Its cylindrical rollers and wedge action provide a safe door-stop type action that increases stopping power as torque increases.

In addition, FitzSimon says the entire system was upgraded to ensure it would handle the 500 hp rating. All the pulleys were changed and the bearings were replaced with larger spherical roller bearing pillow block units.

After operating 20 hours a day, six days a week in the first six months since it was installed, the new drive system has proved to be operating reliably. CEMEX Production Manager Christopher Iaccio says the new system eliminated the previous problems and has been operating without incident since it went online.