Drives in flour mills
April 01, 1998
by Teresa Acklin
Various methods are available to transmit power for equipment operation.
By David Sugden
A drive is defined as the input of energy or power to impel or force a machine to function in a rotary, linear or reciprocal form.
In bygone times, the action of grinding wheat or any other grain was caused by a number of prime movers and energy sources, including oxen, water, wind and steam. Though some of these sources are still in use today in some parts of the world, the main source of power today is electricity.
Before any kind of movement or motion of a machine can take place, generation of electricity must take place. On typical national scales, such generation is variously derived (extension of the word drive) from oil, gas, coal, wind farm, hydro-electric (water), tidal systems or nuclear power. In remote areas, diesel engines are frequently found to supply electricity to mills.
Previously, it was common to operate commercial mills with one prime mover, whether it be a steam engine or a large electric motor. Placement was made to drive line shafts on different floors by means of multi-grooved pulleys for ropes or crowned pulleys for flat belts, thereby providing energy for the entire plant to function.
The prime mover covered the complete mill from intake of grain to output of finished products. All machines within the mill were driven largely by leather belts and counter shafts before the advent of man-made composite materials.
Today, we are accustomed to working with electric motors, in all shapes and sizes, dedicated to a particular machine. Many motors collectively, rather than a single source, now serve as the prime movers or motive forces.
The majority of motors operate on direct current at relatively high voltage of around 400 volts, depending on the local supply. Another variety uses alternating current for low horsepower or kilowatt at somewhere between 110 and 240 volts. Very large motors of as much as 3,000 volts are not common in mills, but do operate in high capacity elevators (silos). Regardless of the type of input, the motor's output, in terms of speed of rotation, needs careful design for the purpose intended.
Many types of drives are in operation, and the particular design and application is very much a question for engineers to recommend to the miller with options. Reliability and ease of maintenance are close to the top of any list of necessities.
Flat belts, constructed of man-made composite materials, are widely used on crown pulleys from the motor output drive to the driven end at usual ratios of up to 4-to-1. One flat belt type capable of transmitting a large amount of power has a chrome leather face with nylon backing.
The flat belt is continuous and specially joined after careful measurement with a 2% or so reduction in length, which allows for natural stretch. The belt is relatively narrow and can be seen driving rollermills, even pneumatic fans. Other materials are also effective.
V-belts carry out a similar job. Because of greater contact surface, this method is even more efficient. The downside is the need for specially grooved pulleys, both for the drive and driven ends, for top performance. Fire-resistant materials are also desirable. The greater the power to be transmitted, the more V-belts come into their own.
Geared motors are used to reduce output shaft speeds to that required for the driven end. Ratios, achieved by means of a train of gears within a gear box, can be of the order of 20-to-1 or more; for example, a 1,400 revolutions per minute motor is capable of driving a screw conveyor at 70 rpm. Direct coupling of geared motors to machines is currently popular because it eliminates the need for external transmission, such as V-belts. In case of malfunction, safety devices include overload trip outs and shear pins.
Chains and sprockets are excellent and versatile non-slip final drives, both at high and low speeds and for power transmission. They are used in applications ranging from rollermill differentials to the final drive of a slow screw bin discharger.
Single-sided timing belts, similar in concept to those used for motor car cam shafts, connect motors to some sifter driven shafts or are used to run feed rolls made by some rollermill manufacturers. The benefit is that they also do not slip.
Torque arm gear boxes are occasionally encountered for rollermill differential purposes, rotary air seals and bucket elevators. The particular value is that they are easily fitted with a spring-loaded limit switch in case of trouble, which acts much faster than an overload trip. The other benefit is that a precision drilled attachment is not necessary. But they are not aesthetically attractive.
Stepper motors, usually coupled by a chain and sprocket at horsepowers of less than 1, facilitate the remote adjustment of powder treatment feeders and rollermill roll gaps. This is accomplished by electrical signals commanding the motor to move both the drive and driven shafts in minute steps in order to open or close by degree. In the case of a rollermill, stepper motors can operate both the left and right hand-wheel control mechanism by fractional steps of less than 10 microns.
Inverter motors are infinitely variable in output speeds by means of changes in electrical frequency. Although not cheap, they find applications in many areas, such as bin dischargers, powder additive feeders, bran finishers, flake disrupters, sifters and so on. Inverter motors can be controlled and set remotely, bringing automation and programming advantages as can be readily imagined.
Hydraulic drives have never been in full vogue or popularity, but they are an effective option. Hydraulic systems, using the controlled use of oil pressure, contain an oil pump with valves that can be infinitely adjusted from open to shut, providing variable speed motion. One mill years ago had all its main machinery driven by hydraulics. The attraction was the ability to adjust the correct speed to mill different wheat for best performance.
One problem with hydraulics, though rare, is the possibility of a burst oil pipe in a flour mill. Another potential drawback in extreme climates is viscosity variation of oil because of temperature changes, which leads to inconsistent operation.
Double-sided timing belts are employed by one rollermill manufacturer in place of gears or chains between the fast and slow main rolls. The particular benefits include reduced heat and noise, no oil requirement and faster main roll changing. Maintenance costs are therefore low.
All drives need maintenance, of course. When it comes to belts, a visual inspection under load is one of the daily items not to be overlooked.
Gear boxes attached directly to motors and/or machinery ideally require inspection according to a regular planned schedule, if only because it is not possible to see inside during operation. The manufacturer's operating and spare parts manual will provide the necessary guidance. Motors and their various starting and overload protection systems can be monitored in a similar way.
David Sugden, independent consultant to the grain industries, may be reached at The Coach House, Killigrews, Margaretting, Ingatestone, Essex CM4 0EZ, U.K. Tel: 44-1245-352048. Fax: 44-1245-251162.