High Torque, Low Speed Wins

The advantages of high-torque, low-speed hydraulics drives for belt and apron feeders are revealed in study of an application from Hgglunds Drives Inc. Consultant Lazlo Toth has recommended and installed the drives on ball mill feed conveyors and other applications since 1969 and has, to date, received no negative feedback on any of them. All of the reports he has received indicated that the installations were successful.

A feeder is a device used to control the gravity flow of bulk solids from storage, such as a bin or a stockpile. For this type of application it is of utmost importance to select a proper drive that can fulfil all the demands put on it.

High starting torque capacity, variable speed and the ability to handle frequent starts and stops are some of the demands that the drive must be able to meet. During the starting of a feeder, the breakaway torque may reach as high as 200% of the running torque. The accelerating torque is usually set to protect the carrying element of the feeder, for example, like the belt feeder and carrying chain of the apron feeder (figure 7).

In normal mode, feeders are running at the operating speed necessary to withdraw the required capacity from the material storage, with minor variations such as changes in the bulk density, operating commands for feed control, and incidental changes in the material flow out of the opening above the feeder. The allowance for this speed variation is plus/minus 25% to 50% of the normal operating speed.

Major change in the operating speed comes from adding or stopping parallel feeders and corrections for severe material flow problems, like runaways or blockages in the discharge opening.

There are three major types of drives used for feeders and low-speed conveyors that require variable-speed operation, says Toth.

The electro-mechanical drive is a combination of an electric motor and a gear reducer. For speed variation, in most cases variable frequency type, a speed controller is used. The electric motor is running at about 50% of full speed. The speed controller is located in the electric room (figure 1).

Another system is the medium-speed hydraulic motor combined with a back-shaft type gear reducer (the gear reducer is not required for conveyors). The electric motor is running at full speed at all times (figure 2). A medium-speed type hydraulic motor is combined with planetary type gear reducer. The reducer is shaft-mounted type and available as a special unit with the hydraulic motor (figure 3). The hydraulic motor is connected to the power unit with flexible hoses and tubes. The power unit is located at a convenient place near the equipment.

With a low-speed, high-torque motor, the setup is a shaft-mounted type and the speed variation is from 0 to 50 rpm. The electric motor runs at full speed at all times. The hydraulic motor is connected to the power unit with flexible hoses and tubes. The power unit is located at a convenient place near the equipment (figure 4).

In the selection of the size of the drive, allowance must be made to overcome the required starting torque of the feeders. High shear force will increase the starting torque of apron feeders a minimum of 100% — and 50% to 75% for belt feeders.

Coarse ore feeders withdrawing materials of greater than 305 mm in size may require additional starting torque (figure 5). In addition to starting torque, allowance should be made for filling pressure (figure 6).

Why use low-speed hydraulic drives?

Low-speed hydraulic drive systems are often chosen for several reasons, including space, maintenance, high starting torque, frequent starting and dependability.

Space: Many feeders are installed where space is limited. The hydraulic motor, compared to the electro-mechanical drive train, is small and requires minimum space, while an electromechanical drive covers most of the feeder on one side, allowing inadequate space for maintenance. The drive side of the feeder is exposed with a hydraulic direct drive. The power unit is conveniently located where space is available.

Maintenance: Because of the available space and relatively small components, maintenance work is not a problem. In addition, the hydraulic motor is easily removable from the head shaft if required. All components of the drive system are handled without difficulties. Handling the large drive reducer of an electro-mechanical drive in restricted areas like tunnels and mines requires special procedures and handling equipment.

High starting torque: The hydraulic drive uses the breakdown torque of the electric motor to start the feeder, and this torque is higher than the pull-up torque used by an electro-mechanical drive. Today we only able to estimate the starting torque of the feeders and this reserve of additional torque, if needed, is appreciated by the operators.

Frequent starting: There is no limit to the number of consecutive starts of the feeder when equipped with a hydraulic direct drive. Additionally, the feeder may run at the lower limit of its speed range constantly, without overheating the electric motor. This feature is not part of an electro-mechanical drive train.

Dependability: With routine maintenance, the hydraulic drive is very dependable.

For more information, contact Hgglunds Drives Inc., Burlington, ON, at info@ca.hagglunds.comor visit www.hagglunds.com,or use the reader reply card number below. Hgglunds was recently acquired by Bosch Rexroth AG.

Reader Service Card No. 401