Gear Drive Mounting Options in Conveyor Applications

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conveyor applications

couplings

gear drive

Gearbox

There are several options for mounting a drive in conveyor applications. Here are the most common types and the pros and cons of each.

In large drive applications such as mining and aggregate applications, you have several mounting options for connecting the low-speed shaft of the gearbox to the gear drive's head shaft pulley. Here, we’ll review the three common options for conveyor applications, including Rexnord’s MCF couplings.

Mounting the Gear Drive to the Conveyor

In conveyor applications, the most common method for mounting a gear drive to the conveyor head shaft pulley is to support the motor through a bell housing (also known as an Alignment Free Drive) or a small platform (commonly referred to as a swing base) that supports the gear drive and motor, as shown at right.

With the motor supported, the next decision a designer needs to make is how they want to connect the low-speed shaft of the gearbox to the head shaft pulley of the gear drive.

Conveyor Drive with a swing base and TA Taper bushing to mount the gear drive on the head shaft pulley of the conveyor(top). Gear drive with a bell housing and an MCF shown below.

Conveyer-Drive-(2).jpg

Connecting the Low-Speed Gearbox Shaft to the Gear Drive Head Shaft Pulley

There are three standard options for connecting the low-speed shaft of the gearbox to the gear drive's head shaft pulley. Here are the advantages and disadvantages of each coupling selection for conveyor applications.

Straight Hollow Bore with a Key

The first coupling option is to use a straight hollow bore with a key. Of the three options, this is the most cost-effective, and is commonly used on head shaft pullies that are 4” or smaller. The drawback of this coupling option for conveyor applications is that it’s difficult to mount due to the tight tolerances between the conveyor pulley shaft and the gear drive hollow bore. These tight tolerances will also make it difficult to remove the drive from the head shaft pulley after the unit has been in service. Dirt, debris, and wear material can effectively weld the conveyor pully shaft to the gear drive hollow bore. Because of this challenge, the head shaft pulley will often need to be cut off to dismount the drive for service or replacement.

Hollow Shaft with a Locking Mechanism

The second method for connecting the shaft of the gearbox to the head shaft pulley of the gear drive is to use a hollow shaft with a locking mechanism. This second coupling option is slightly more expensive than using a straight hollow bore, due to the drive being shipped with a locking mechanism on the low-speed shaft (such as a Tollok shrink disk or a Rexnord TA taper bushing), and a tighter tolerance required on the conveyor pully shaft.

This coupling is the method of connection commonly used on conveyors with head shaft pulleys in the 4-8" range. The advantage of using this connection option is that the locking mechanism makes it more likely the drive is removable without cutting the head shaft pulley of the conveyor, but removal is far from guaranteed. Similar to the hollow bore shaft mounting option, this low speed shaft configuration is also susceptible to dirt, debris, and general fretting that can effectively weld the conveyor pulley shaft to the hollow low speed bore of the gear drive. An additional disadvantage of this method is a longer assembly time due to the shrink disk installation requiring a very specific tightening pattern and sequence. This can cause the drive to slip on the pully shaft and fail prematurely if not followed correctly.

To offset some of the disadvantages of the shrink disk-related problems, Rexnord has developed a TA taper bushing that is the easiest to put on and least likely to fret onto a shaft. The TA taper bushing offers a significantly simplified solution by allowing the bushing to be mounted on the conveyor drive pulley prior to drive installation, only requiring one nut be tightened. Due to the use of dissimilar metals, special coatings, and a tapered design, this is the hollow shaft solution most capable of removal after prolonged use without cutting the head shaft pully of the conveyor. This option is also used in the 4-8” conveyor head shaft pulley size range.

MCF Rigid Mount Coupling

The third option for connecting the gearbox shaft to the gear drive pulley is to use a rigid mount coupling like the MCF. Rexnord’s rigid mount coupling solution, the MCF, is typically used on head shaft pullies that are greater than 8” in diameter. The design of an MCF Coupling guarantees the ability for the drive to be removed from the head shaft pulley of the conveyor but does so at the cost of mounting the drive further away from the head shaft pulley bearing. This causes the need for larger supporting bearings and a larger supporting structure.
MCF couplings were designed with the needs of mining and aggregate industries in mind, although they are suited for other applications too. The durability makes them ideal for most processing and mining phases including extraction, handling, and preparation of coal, copper, iron ore, potash, and other such materials. If a rigid shaft connection, as opposed to a hollow low-speed shaft connection, is needed or preferred, MCF couplings are a reliable choice.

The MCF coupling was specially designed as an add-on for V-Class drives, but it can also be retrofitted to A, Y, and other legacy units. The engineered design of the MCF makes it easy to install and remove. Of the coupling options, the MCF is the simplest to mount and can always be removed from the head shaft pulley. This design makes the MCF a cost-effective option in the long run.

It's also important to note that Rexnord now offers an RSB (Rough Stock Bore) Hub version of this coupling, meaning the coupling is available half or full without a bore in it. Customers can put a bore in at the time of installation (guidance on dimensions are available through your Rexnord consultant and in the catalog). This option allows you to have a spare drive on hand for future use. The RSB version is also a good option if you've wrecked your head shaft pulling a drive off and don't know the exact dimension of your new head shaft.

Engineering Should Review Your Coupling Selection

Unlike other coupling selections where the coupling is just transmitting the application torque, a rigid coupling is holding up the entire weight of the gear drives while also transmitting all the application torque. This means that not only is the maximum transmitted torque an essential factor, but also the shear and overhanging moment forces created by the drive assembly weight (as shown below). Due to these added performance requirements it's critical that your MCF selection is reviewed by engineering to ensure it fits your application. Rexnord has provided a form to help collect all of the required application data for the selection of our MCFs in our MCF catalog (document number GR2-007 pages 17 and 18), but all MCF applications need to be reviewed by engineering due to the critical nature of this connection.

Pictured: Free body diagram of the forces transmitted through a rigid mount coupling

Ryan Schuller-Rach

Ryan is the Commercial Operations Leader for Regal Rexnord's Industrial Powertrain team. He is a Professional Engineer in the state of Wisconsin and has been with Rexnord Power Transmission since 2011, where he has held leadership roles in new product development, product management, and engineering technical services. Ryan graduated from Bradley University with a Bachelor of Science in Mechanical Engineering and a Masters in Business Administration from Marquette University.