Selecting the Optimal Conveyor Drive

ABSTRACT

This paper suggests several mechanical, electrical, and software ideas that will enable conveyor designers to optimize conveyor reliability and "maintainability" without the need for large investments in equipment and design time.  It introduces the concept of internally powered conveyor belt drives and offers generally applicable belt drive design software. 

1.0 INTRODUCTION 

Architect engineers and OEM designers face the same two challenges – "One, how do I offer my client the most reliable conveyor system at a competitive price?  And two, how can I minimize ongoing system expenses for my client?" 

A large array of equipment choices is available and the Internet seems to offer an unlimited amount of technical information.  Are there a simplified list of components and straightforward methodology for the conveyor designer to use?  Yes. 

The technology is well established and design guidelines are nationally recognized.  Design software has been developed and is readily available. 

This paper will serve as an introductory summary rather than an exhaustive treatise on conveyor design.  It also suggests the concept of internally powered conveyor belt drives for consideration.  See figure 1. 

The reader should refer to the latest edition of "Belt Conveyors for Bulk Materials" by the Conveyor Manufacturers Association for a comprehensive conveyor design guide [1].

Fig. 1:  Two 7.5 HP motorized pulleys in a congested area of a mobile crushing and screening plant installed in Ottawa, Canada in 1993.

 

 

 

 

 


Fig. 2:  This food-processing conveyor illustrates the need for inherent conveyor drive safety.

 

* Presented at Materials Handling Industry of America "NA2002 Show & Conference," Detroit, MI, 4/10/02.

2.0 THE PROBLEM 

The engineering challenge is to design a system to move either unitized or continuous products on a belt from point A to point B.  See figure 2.  Drive power is available either electrically or hydraulically.  In general, hydraulic conveyor drives are limited to highly mobile, low power conveyor applications.  This paper addresses both mobile (figure 3) and fixed (figure 4) systems over a broad spectrum of power requirements and presents information on electric motor drives only. 

The designer must know or make reasonable assumptions on the following parameters:

  • Product flow rate

  • Belt speed

  • Conveyor Length

  • Product Size

  • Lift Height

  • Type and size of belt

  • Type of belt support

  •  

Then the designer must make the following system control choices:

  • Is flow continuous or intermittent

  • Is belt speed fixed or variable

  • What is conveyor duty cycle

  • What are extremes of process flow

  • What extremes of ambient environment

  • What safety requirements apply

  •  

After these parameters and control choices are established, the designer may determine horsepower requirements and select mechanical and electrical components. 

3.0 THE SOLUTION 

To determine required HP the designer simply calculates required belt pull at the specified belt speed.  A variety of factors have been empirically developed to simplify the determination of belt pull, as described below. 

Some of these factors include:

  • Belt/slider bed coefficient of friction

  • Pillow block and idler roller bearing friction

  • Skirt board friction

  • Friction due to belt flexure

Fig. 3:  The first of six mobile crushing and screening plants, this rig uses motorized pulleys to drive screen feed belt and exposed drives on product take away belts [2].  Due to space limitations, screen feed used two 15 HP drives (one at head and one at tail).

 

Fig. 4:  This major package handling system uses motorized pulleys working with variable frequency drives to minimize maintenance expense and maximize long term system "up time."

1 HP equals 33,000 ft-lbs per minute.  When belt speed (ft/min) and belt pull (lbs) are known, HP may be specified (i.e. belt speed x belt pull = required HP.)  For example, if operational requirements call for a 3,300 lbs belt pull at a belt speed of 275 fpm, then 907,500 ft-lbs per minute, or 27.5 HP, would be required. 

However, prior to specifying components, the designer must allow for inefficiencies and specify HP accordingly.

Fig. 5:  Exposed AC squirrel cage induction motor and right angle helical gear reducer are used at this package handling facility.

 

  According to CEMA, the following external power transmission mechanical efficiencies are to be expected [3]: 

  • Medium-ratio worm gear speed reducer 70%

  • Triple reduction helical gear reducer 95% (figure 5)

  • V-belts and sheaves 94% (figure 6)

  • Roller chain and cut sprockets 93%

 

Internally powered belt drives (figure 7) consist of oil cooled direct-coupled motors and gearboxes mounted inside the pulley shell.  Gearboxes are helical/spur or helical/helical/spur in two and three stage reductions respectively.  See figure 8.  These drives offer 94 to 97% efficiency. 

Therefore, if the designer chooses an external helical gear reducer as mentioned above, he should specify a 30 HP motor at sea level (27.5HP/0.95 = 28.95 HP, choose 30 HP.) 

If the designer chooses this helical gear reducer and a V-belt and sheave, he should specify a 35 HP motor at sea level ((27.5HP/0.95)/0.94 = 30.78 HP, choose 35 HP.) 

If the designer chooses an external worm gear reducer as mentioned above, he should specify a 40 HP motor at sea level (27.5HP/0.70 = 39.3 HP, choose 40 HP.)

Fig. 6:  This mobile crushing and screening rig used an exposed motor with a helical gear reducer and V-belt/sheave arrangement to drive the product take-away belt and a motorized pulley to drive the elevating screen feed belt (shown in stowed position for transport.)

  As can be seen in these examples, the type(s) and number of gear reducers used in a system will have a direct impact on the initial cost and operating cost of a conveyor system. 

A 40 HP motor is more expensive than a 30 HP motor.  It will also consume more electrical power to do the same work.  Assume that electrical power costs the operator $0.05/ kW-Hr and that the system runs two shifts per day, six days per week, 51.5 weeks per year.  

The worm gear will require 145,000 kW-hrs per year at a cost of $7,247 per year.  (39.3 HP x 0.746 kW/HP x 4,944 hrs/yr = 145,000 kW-hrs/yr.) 

The 30 HP motor will only require 106,774 kW-hrs per year at a cost of $5,339 per year. 

The difference in initial cost of the two drive systems is dwarfed by the annual operating cost difference.  In ten years, the worm gear operating cost premium will equal nearly $20,000. 

Therefore, the selection of belt drive components has a significant impact on short-term and long-term costs.  Additionally, the designer must compare the cost of downtime with the type of drive system specified.  Typically, the higher the cost of downtime, the more likely will be the need for robust, reliable drives and an investment in spares components.  See figure 9. 

4.0 DRIVE COMPONENT CHOICES 

Although a large variety of conveyor belt drives and brands are available, the basic motor, power transmission, and control components can be summarized as follows:

 

  • Single or multiple power transmissions

  • Internally or externally driven belt pulley

  • Constant or varying belt speed

 

Multiple transmissions (e.g. helical gearbox with a V-belt and sheave arrangement) have the advantage of flexibility (e.g. sheaves may be easily changed to vary speed.)  However, they sacrifice efficiency as described above.  See figure 10.

Fig. 7:  This motorized pulley protects internal 15 HP motor and two-stage gearbox from harsh environment at this lake dredge operation. 

Fig. 8:  Components of motorized pulley.  From right to left: junction box, non-rotating shaft, design C (Class F) AC motor, helical gears (1st stage of gear reducer,) spur gears (2nd stage of gear reducer,) and non-rotating shaft.  Drip lips welded to inside of pulley shell circulate oil to lubricate all moving parts and cool motor.

 

Fig. 9:  Motorized pulley is used in "under slung" drive section of this parcel-elevating conveyor.

External drives have the advantage of ease of component replacement in case of motor or gearbox failure.  However, compared to internal drives, they expose the drive elements to the environment and must be guarded for safety reasons.  See figure 11.

 

Constant speed belt conveyors with power requirements greater than 1/3 HP are usually driven by three phase AC squirrel cage induction motors.  See figure 12.  Lower powered constant speed belt conveyors are frequently driven by single-phase AC motors. 

 

There is a strong trend for variable speed fixed conveyors to be driven by a three-phase AC motor working with a variable frequency drive (VFD.)  The use of DC motors to drive variable speed belt conveyors is declining because of the lower motor and control cost and high reliability of the AC motor/VFD concept.  See figure 13.  Designers of long overland conveyors have also adopted the AC motor/VFD concept to reduce electrical power consumption [4].   See figure 14. 

5.0 SELECTING BELT DRIVE & CONTROL SYSTEM 

Two examples will illustrate belt drive optimization.  In both examples the designer has chosen an internally powered drive.  In case one, the drive was chosen to maximize system reliability and operator safety and to minimize maintenance expense in a package handling system.  In case two, the drive was chosen due to space limitations in a mobile sand and gravel processing plant.

 

Case One:  This Denver Colorado package handling system must provide two process rates on this indoor horizontal transfer and manual sortation conveyor.  See figure 15.  Customer specifications require that all belt conveyors use steel slider beds to support the carrying side of the belt.  VFD technology is well accepted at this facility.  Belt width, conveyor length, and both processing speeds have been set by plant layout, package sizes, and processing rates.  VFD must be located in motor control center 100 feet from conveyor drive.

Fig. 10:  Product take-away conveyor is driven by 10 HP AC motor, V-belt/sheave arrangement, and in-line helical gear reducer.  By changing sheaves operator may change belt speed mechanically.  Note OSHA-required guard over sheaves and V-belts.

 

Fig. 11:  Motorized pulley protects 15 HP AC motor from abrasive material at this German limestone quarry.  Note that a motorized pulley may be mounted in a screw take-up to conserve space.

 

Fig. 12:  This typical Canadian stone producer generates electrical power with a portable generator to drive all conveyors and stone crushers.

The designer must:

  • Determine optimal belt drive HP

  • Select correct control options

Design parameters are:

  •  Conveyor length: 137.5'

  •  Belt width:  60"

  •  Belt weight: 5 lbs/ft

  •  Avg. package wt: 140 lbs

  •   Peak rate:  3,000 pkgs/hr

  •   Normal rate: 1,950 pkgs/hr

  •   Peak belt speed: 100 fpm

  •   Normal belt speed:  50fpm

  •   Slider bed/belt coefficient of friction: 0.32

 

As described below, calculated belt pull is 3,300 lbs at 100 fpm (330,000 ft-lbs/min) and 4,500 lbs at 50 fpm (225,000 ft-lbs/min.)

 

Therefore, required drive power at high speed is 10 HP and required HP at low speed is 6.8 HP.

 

State-of-the-art motorized pulleys will provide no less than 95% of full load torque within the allowable frequency spectrum (i.e. 12 to 66 Hz.) 

 

The designer should select a 15 HP motor with a gearbox yielding a belt speed of 100 fpm at 460volts/3 phase/60 Hz.  When running on a VFD at 30 Hz, this motor will be equivalent to a 7.1 HP motor.

 

Motors should be derated by 2.5% in Denver (at an altitude of 5,000 ft above sea level.)  Note that the motor will be adequate at both speeds at this altitude.

 

Designer should select an appropriate VFD for the 15 HP belt drive motor.  However, note that since the VFD must be located more than 30 feet from the drive motor, the designer should also specify an output filter on the VFD.  The reason is that resonant frequencies can occur on the line between the VFD and the motor causing a voltage spike and damaged motor.

 

 

 

Fig. 13:  All belt conveyors at this major package handling facility are driven with variable frequency drives.  This allows the operator to choose various processing speeds while minimizing the number of spare conveyor drives.

 

  Fig. 14:  To optimize the amount of material carried on a 6 mile long German overland conveyor, a laser scanner measures material cross section on the belt while another sensor measures belt speed.  The control system continuously interprets this data in order to optimize the material fill factor on the overland conveyors.  The speed of the 1,200 to 1,700 HP conveyor drive motors is varied as a function of the volume of material on the belt.

 

 

 

Case Two:  The manufacturer of this mobile bulk material sizing system needs to install the smallest diameter pulleys and narrowest belts to process 1,000 tph of material.  The system will require three conveyors.  See figure 16.  To minimize the number of spares, the same drive type will be used on three different conveyors.  One conveyor elevates material into the system and two conveyors discharge the product downstream of sizing.

 

The designer first must determine which conveyor "governs" (the elevating belt carrying the full process rate, in this example.)  Next the designer must do the following:

 

Select Belt Width:

  •          With bulk density and belt speed fixed, select width to produce rate Q, not exceeding "standard edge distance"

  •          Width must be > 3x max lump for 200 surcharge and > 6x max lump for 300 surcharge

  •          Width must be wide enough to prevent loading chute and skirtboard jamming (i.e. > 3x to 5x max lump)

 

Select Belt Speed:

  •         With bulk density and width fixed, select a speed to produce rate Q, not exceeding "standard edge distance"

  •          For dusty material, select speed to minimize fugitive dust emissions

  •          For heavy sharp material, select speed to protect belt and chute lining

 

Calculate Required HP [5]:

  •          With belt width and speed fixed, select conveyor components and calculate belt tension required to overcome gravity, friction, and momentum using:

Te = LKt (Kx + KyWb + 0.015Wb) + Wm(LKy + H) + Tp + Tam + Tac

 

  • Calculate power required to drive belt using:

HP = (TeV) / 33,000

Fig. 15:  In AC motor/VFD systems with two belt speeds, such as this one, it is essential to specify enough HP.  HP is linearly proportional to frequency.  A "constant torque" 10 HP 460volt/3phase/60Hz motor will only provide 5 HP at 30 Hz.

 

Fig. 16:  Fisher Industries (General Steel and Supply) of Dickinson, North Dakota used internally powered conveyor belt drives in this mobile bulk materials sizing system to minimize space and spare parts requirements while maximizing system reliability.

  Finally, the designer may select the drive:

  • Select motor/transmission to match design speed and provide enough power

  • Recalculate effective tension and required power, using selected speed

  • Select drive pulley diameter, insuring that wrap factor and belt life are acceptable

  • Check cross section of material on belt, insuring that edge distance is acceptable

  • Check material trajectory, insuring that transfer chute will not plug and material will drop at desired location

  • Specify motor/transmission, insuring that package yields good belt design, fabrication, and end user value

 

6.0 CONVEYOR DRIVE DESIGN SOFTWARE

 

Complimentary design tools in the form of software packages have been provided to bulk handling design engineers since 1994.  See figures 15 and 16.  This software was presented in several seminars throughout the USA in 1995 and has been in use since that time.  It is available free of charge to any bulk handling design engineers.

 

Based on Conveyor Equipment Manufacturers Association (CEMA) guidelines as well as other reliable technical sources, this software calculates conveyor drive horsepower requirements and checks material trajectories and belt fill cross sections.  It incorporates allowances for feeder belt drives, slider beds, cleated belts, and traveling trippers. 

 

The purpose of the software is to enable designers to quickly calculate drive power requirements while optimizing the belt speed versus belt width decision.  This allows designers to choose the smallest drives to fit their particular application.

 

In 1998 yet another program was developed to assist package-handling designers calculate HP.  See figure 17.  This, too, is also available, free of charge, as a Microsoft Excel spreadsheet [6]. 

Fig. 15:  Conveyor design software enables designer to optimize bulk material discharge trajectory.  Program plots selected speeds for various pulley diameters and conveyor inclinations.

 

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Fig. 16:  Conveyor design software plots material cross section for various troughing idler angles.  It enables designer to optimize belt speed vs. belt width for any bulk density.

 

   

7.0 SUMMARY

 

Today's conveyor system designer has a large number of drive options available.  Optimizing the drive and drive control system requires striking a balance among several items including: the initial equipment cost, the costs of maintenance and operation, and system "up time," flexibility, and safety. 

 

A variety of conveyor programs are available to simplify the designer's tasks and reduce system design time.  Externally and internally powered conveyor belt drives are available.  This paper highlighted key design elements and presented some of the most important features of motorized pulleys.

 

 

 

 

 

Fig. 17:  Conveyor design software calculates belt pull and HP requirements for package handling conveyors.  Program is valid for horizontal or inclined, accumulating or non-accumulating, and slider bed or roller conveyor systems.

 

This tutorial was written and provided by Michael J. Gawinski, P.E. of Interroll Corporation. Interroll is a group of companies with global operations in the growth markets of materials handling, distribution and automation. They develop, produce and distribute components and subsystems for manufacturing and logistics installations. For more information on Interroll, please visit their website at www.interroll.com.

8.0 REFERENCES  

1. Belt Conveyors for Bulk Materials, Conveyor Manufacturers Association, Baltimore, ND, 1993. 

2. Gawinski, M:  Motorized Pulleys Solve Materials Handling Problems in North America, Bulk Solids Handling Vol. 21 (2001) No. 2. 

3. Belt Conveyors for Bulk Materials, Conveyor Manufacturers Association, Baltimore, ND, 1993, page 189. 

4. Kohler, Sykula, and Wuschek, V.:  Variable-Speed Belt Conveyors Gaining in Importance, Surface Mining Vol. 53 (2001) No. 1, S. 65 – 72. 

5. Belt Conveyors for Bulk Materials, Conveyor Manufacturers Association, Baltimore, ND, 1993. 

6. Conveyor Components, Interroll Corporation, Wilmington, NC, December 2000, page 156.