How to rotate a conveyor belt forward and reverse｜Forward/Reverse Rotation CV Selection

Here, you should be aware of the following when you have a purpose such as "to make a workpiece run back and forth on the layout of a conveyor line" or "to discharge a defective workpiece by running it in reverse only when it is defective"."How to rotate the conveyor belt forward and backward."　Here is a note about the

It is not uncommon for such requests to arise in the conceptual design of automatic machines.　　At first glance, it seems as if it could be easily accomplished by simply rewiring the motor.　　However, when trial operation is started, problems frequently occur, such as "the belt quickly becomes uneven and rubs against the frame," "the workpiece slides only in reverse," or "the motor stops due to abnormal heat generation," and have you ever been troubled?

Many Web sites tend to simply conclude that "meandering prevention guides should be installed.　　However, surprisingly little information is available that explains why meandering occurs when reversing, its mechanical mechanism, the difference in tension variation depending on the drive system, and even calculations and safety measures based on JIS standards.

This article is more than just a product introduction,Engineering rationale for "why the selection is necessary"The course will delve into the following topics and provide a comprehensive explanation of the path to solving issues faced in the field so that designers can make specification decisions with confidence.

Basic theory of how to rotate a conveyor belt forward and reverse

Conveying problems caused by changes in effective tension

When designing a belt conveyor, it must first be understood that the "tension distribution" on the belt changes dramatically between forward and reverse rotation, mechanically.　　Typically,The conveyor transmits power when the drive pulley pulls the belt by frictional force.　　In this case, the belt on the side toward the drive pulley is strongly tensioned (tension side: T1), and the belt on the side being fed from the drive pulley is loosened (slack side: T2).

In a design that assumes only unidirectional operation, the position of the drive pulley (usually the head side in the transport direction) is determined so that the "carrier side" on which the workpiece is placed is always the "tension side.This causes the belt to carry the workpiece in a panned and tensioned position.　　However, when the same mechanism is reversed (reversed rotation), this mechanical relationship is completely reversed.The belt on the carrier side, which has been pulled until now, is "pushed" out of the drive pulley when reversing, and changes to the "loose" side.The first is the

When the tension on the carrier side decreases, the belt tends to deflect (sag) due to its own weight and the load of the workpiece.　　This deflection not only deteriorates transfer accuracy, but also increases contact resistance between the belt and guide rails, causing premature wear.

Furthermore, the belt exhibits rippling behavior, which directly leads to problems such as workpieces tipping over or losing their posture.　　Therefore,When planning bi-directional conveyance, it is essential to design a mechanism that can maintain the effective tension required for conveyance in either forward or reverse direction, or to set the tension with a sufficient safety factor.It is.

Causes of frequent slips during reversals

A particular concern for designers of conveyors that operate in forward and reverse rotation is the "slip" that occurs between the drive pulley and the belt.　　The limit at which a belt conveyor can transmit driving force is described by Euler's belt formula.

Important,The fact is that the force that can be transmitted depends on the "slack side tension (T2)".　In other words, when the tension on the slack side approaches zero, no driving force is transmitted, no matter how high the output of the motor, and only the pulley idles.

As mentioned above, when a conveyor with a general head-driven (tip-driven) system is reversed, the carrier side on which the workpiece is loaded becomes the "slack" side.　　If a heavy workpiece is attempted to be conveyed in this condition, the tension required to overcome the frictional resistance between the belt and the slider bed (or rollers) is insufficient, resulting in slippage.

In order to avoid this phenomenon, we have seen some sites where the initial tension is set extremely higher than usual and adjusted so that the slack side tension is secured even when reversing.　　However, this is only a symptomatic treatment.　　Excessive initial tension always applies excessive radial load to the pulley bearings and motor shaft, which significantly shortens the life of the machine.The fundamental solution to prevent slipping is not a simple increase in tension, but rather "proper selection of the drive system," which will be discussed later.

Crown pulleys alone do not prevent meandering.

The "crowning" process, in which the center diameter of a pulley is larger than its ends, is widely known as a measure to prevent meandering of the belt.　　This is a simple and effective self-aligning function that takes advantage of the belt's physical tendency to move toward the direction of higher peripheral velocity (larger diameter).

For a conveyor with unidirectional operation, simply employing the appropriate crown pulley can absorb some installation errors and belt quirks and keep the belt in the center position.

On the other hand, meander control that relies solely on crown pulleys is extremely difficult and risky for conveyors that repeat forward and reverse rotation.　　The main reason for this is that minute distortions in the conveyor frame and alignment (parallelism) errors in the pulleys and rollers act as completely different vectors in different directions of rotation.

For example, even if the balance is maintained by accident during forward rotation, the moment it reverses, the balance is lost and the belt snakes to one side at once, contacting the frame and damaging the belt ears, which is a frequent occurrence.

In addition, if the belt has been used in forward rotation for a long period of time and has become "blended" or "wound", suddenly reversing the belt may release the residual stress inside the belt and cause unexpected behavior.The crown effect is only a gentle centering action and does not have enough forcing power to instantly compensate for large misalignments caused by sudden tension fluctuations or external disturbances during reversal.Therefore, a prerequisite for bi-directional conveying is the use of a guide mechanism that physically regulates the position of the belt.

Design standards learned from JIS B 8803 standard

In Japanese industrial machinery design, the key to calculating and designing conveyor belts is the "JIS B 8803(Belt Conveyors - For General Use - Calculation Formula)" and other Japanese Industrial Standards.

The standard defines in detail how to calculate the required power and the formulas for calculating the tension applied to each part.

It is important to apply the concept of this standard to the design of bidirectional operations.　　The standard provides a procedure for checking the maximum tension that takes into account not only the power during steady-state operation, but also the "acceleration resistance" and "deceleration resistance" applied during startup and shutdown.　　In particular, when switching from forward to reverse rotation, the deceleration and re-acceleration process can place several times the steady-state load on the belt.

Through a calculation process based on JIS standards, we can quantitatively determine the "minimum tension required to prevent the belt from slipping during reversal" and the "safety factor to prevent the belt itself from breaking (usually about 8 to 10 times).

By selecting motors and belts based on numerical values in accordance with standards, rather than relying on experience and intuition, unexpected problems can be prevented and highly reliable automatic machines can be designed.

Method and model selection for forward and reverse belt conveyor rotation

Why head drive is not suitable for bi-directional transport

The most common type found in the catalogs of MISUMI and other conveyor manufacturers is the "head drive" method, in which the motor is placed at the tip of the conveyor direction.

Because of its simple structure, small number of parts, and low cost, it is the first choice for ordinary one-way conveyance.　　However, the following is a list of the most common types of one-way conveyors,In many cases, this method is not recommended when "forward/reverse rotation" is included in the specifications.

The biggest reason is the "asymmetry of tension distribution" mentioned in the previous section.　　In the head drive, the carrier side is taut and stable during forward rotation (when the workpiece is conveyed toward the motor), but during reverse rotation (when the workpiece is conveyed away from the motor), the carrier side becomes loose.　　The longer the machine length, the greater the elastic deformation of the entire belt, and the more pronounced this effect becomes.

The following table summarizes the differences in characteristics by drive system.

As can be seen from the table, reversing head drive operation is structurally unstable.　　While it may be possible to compensate for this by operation in cases of light loads, low speeds, and infrequent operation (e.g., reversing only for maintenance), it is extremely difficult to adjust when performing regular reciprocating conveyance as an automatic machine, and may compromise the long-term reliability of the machine.

If stability is important, choose an intermediate drive.

The "center drive" method makes the most sense from a mechanical standpoint for bi-directional conveyance.　　This system has a structure in which the motor and drive unit are located in the center of the conveyor length (underside of the return side belt).

The greatest advantage of the intermediate drive is that "the change in tension distribution is nearly symmetrical" between forward and reverse rotation.　　Since the drive is located in the center of the return side, it is easier to maintain the belt on the carrier side (workpiece conveyance surface) being pulled between the pulleys at both ends (both of which are idlers), no matter which direction it is rotated.

The belt is physically less prone to sagging on the carrier side, as seen when the head drive is reversed, enabling extremely stable workpiece transfer.

Another advantage of selecting an intermediate drive type, such as MISUMI's "CVSN" series, is that both ends of the conveyor can have the same shape (e.g., small diameter pulleys or knife edges on both ends).　　This allows for uniform transfer conditions with the equipment before and after the conveyor, greatly increasing the flexibility of the line layout.

Although the initial cost tends to be slightly higher than the head drive, considering the cost of troubleshooting during commissioning and downtime after operation, selecting the intermediate drive for bidirectional operation without hesitation is the most economical and reliable choice as a result.

Why should you choose a belt with anti-snaking cleats?

The use of ordinary "flat belts" without guides should be avoided on conveyors that perform forward and reverse rotation.　The meandering behavior in reverse is difficult to predict, and even if a skilled operator takes the time to adjust the pulleys, there will always be some misalignment after long periods of operation.Therefore, it is essential to select a belt with "anti-snaking cleats (V-guide)".

Anti-snaking cleats are trapezoidal or V-shaped profiles welded to the back of the belt over its entire circumference.　　These fit into the V-grooves machined into the pulleys and slider beds, thereby physically restraining the lateral displacement (movement in the thrust direction) of the belt.　　With this mechanism, even if the direction of rotation changes and the tension balance is about to be upset, the cleats act as guide rails to force the belt to maintain its track.

In selecting a model of MISUMI, specify a type with "anti-snaking" or "with V-guide" specified in the belt specification code (e.g., SVKR, etc.).

This not only frees the operator from the troublesome on-site meandering adjustment work, but also dramatically reduces the risk of the belt coming into contact with the frame and damaging it, or of shavings getting mixed in with the workpiece.In bidirectional conveyors, V-guide is an essential feature that should be considered "standard equipment" rather than an option.

Designed to prevent accidental ride-up of the V-guide

Belts with anti-snaking cleats (V-guides) are a very powerful anti-snaking solution, but they are not a panacea.　　If designed or assembled incorrectly, there is a risk of a serious accident called "ride-up.

This phenomenon occurs when the V-guide lifts off the V-groove of the pulley and rides on the surface of the pulley when the frame is forced to operate with a large deviation in alignment (parallelism or squareness).

Once the guide rides up, the belt circumference is forcibly stretched, resulting in abnormally high tension.　　As a result, the belt can break instantly or the motor shaft can lock up and stop overloading.　　To prevent this, it is important to strictly check the conveyor frame itself for twisting by measuring the diagonal dimensions (tusk hangings).　　The V-guide is only an "aid" and cannot cover all of the machine inaccuracies.

Care must also be taken in selecting the pulley diameter.　　Since V-guide members have a certain thickness and hardness, they have high bending rigidity, and pulleys with extremely small diameters (e.g., knife-edges of φ15 mm or less) will not bend smoothly, causing them to lift off.　　It is the responsibility of the designer to ensure that the "minimum pulley diameter for V-guided belts" in each manufacturer's catalog specifications is adhered to, and that proper tension control is performed to prevent the guides from coming off due to impact during forward and reverse rotation.

Control design of a method to rotate a conveyor belt in forward and reverse direction

Note the 30-minute rating of the reversible motor.

As a drive source for small conveyors, the catalog lists a single-phase motor with the name "reversible motor.　　As the name suggests, this motor is capable of forward and reverse rotation, and has a feature of instantaneous reversal (inching operation) due to its built-in simple brake.This may seem like the best choice for two-way conveyors, but there is a major pitfall in automatic machine design: the "rated time.

Most reversible motors have a "30-minute rating" specification.　　This is,It basically means that the continuous operation time is limited to 30 minutes.　　If the frequency of direction changes is high or continuous operation is performed, the heat generated will exceed the allowable value and the thermal protector (overheat protector) will be activated, causing the motor to stop.

Features of reversible motors (Oriental Motor)

Ideal for applications where forward and reverse operation is frequently repeated. This motor is rated for 30 minutes and can instantly switch the direction of motor rotation. A simple brake is built into the rear of the motor, making it ideal for applications that require frequent repetition of forward and reverse operation.　The motor is rated for 30 minutes of continuous operation, but can be operated for more than 30 minutes depending on the operating conditions (e.g., intermittent operation).

The following table compares the main motor types and their suitability for two-way operation.

Since factory production lines are often designed to operate for long hours, leversible motors that can run continuously for only 30 minutes should be avoided for applications other than intermittent operation (i.e., running while resting).

It is important not to be misled by the name "leversible," but to always check the specification table to see if the product is continuously rated (continuous).

Three-phase induction motor and inverter control

The most standard and reliable configuration for forward and reverse belt conveyor rotation in industrial automatic machines is a combination of a "three-phase induction motor" and an "inverter.　　Three-phase motors are robust in construction and can be operated at continuous ratings, and can be optimally controlled through an inverter.

The greatest advantage of using an inverter is that it electrically controls the frequency to achieve smooth "acceleration and deceleration (soft start and soft stop).　　While normal switching causes reversal with a "thud" impact, setting the deceleration time with an inverter allows smooth stopping and reversing.　　This prevents the workpiece from falling over or slipping due to inertia, and minimizes damage to the belt and mechanical components.

In addition, since forward, reverse, stop, and multi-step speed changes can be freely controlled by external signals, it can easily be incorporated into an automatic operation system linked with a programmable logic controller (PLC).　　Three-phase 200 V motors" can also be selected when ordering conveyors from MISUMI and other manufacturers. Considering the balance between cost performance, durability, and controllability, this combination is the most versatile and solid choice with the least trouble.

Moment of inertia calculation and selection of regenerative resistance

When the workpiece being conveyed on the conveyor is heavy or when the conveyor is operating at high speed, the handling of "regenerative energy" generated during stopping and reversing must be considered.　　When a motor is decelerated, it temporarily acts as a generator, converting kinetic energy into electrical energy to be returned to the inverter side.

Normal gradual deceleration can be absorbed by the capacitor inside the inverter, but rapid deceleration or high frequency reversals cannot be absorbed, resulting in an "overvoltage error (E.OV)" and an emergency stop of the conveyor.　　To prevent this, an optional "regenerative resistor (brake resistor)" must be connected to the inverter and designed to release excess electrical energy as heat.

It is also important to calculate the "load moment of inertia (J or GD^2)" at the design stage and verify whether the selected motor and inverter are acceptable.　　Forcing the motor to stop or reverse in a short period of time will result in insufficient motor torque or overcurrent errors.　　Determine the balance between takt time requirements and physical inertia load, and if necessary, consider increasing the motor capacity rank or changing the reduction ratio.

Involvement risk arising in the inversion section

Finally, the risk of entrapment should never be forgotten in terms of safety design.　　In a normal conveyor with one-way operation, the area where the belt becomes entangled in the pulley is limited to the lower downstream side of the conveyor or the take-up area, which is usually protected by a cover or other means.

However,When the belt is operated in reverse, the location where the belt used to be on the "coming out side" is now on the new side where the belt is "caught".　　For example, the head end of a conveyor is safe in forward rotation, but the moment it reverses, it turns into a hazardous area where fingers and clothes can be pulled in.

The designer should ensure that the operator cannot touch the hazardous point not only in forward rotation but also in reverse rotation,JIS B 9700(Safety of Machinery), you are obligated to perform a risk assessment based on the following.　　Specifically, the gap between the pulley sections at both ends must be controlled to prevent fingers from entering (e.g., 5 mm or less), or additional safety covers must be installed.　　It is also essential to take multiple safety measures in the control circuit, such as ensuring that an interlock circuit is built in hardware or ladder program to immediately stop the machine if a forward and reverse rotation signal is input simultaneously.

Summary of how to rotate the conveyor belt forward and reverse

• Forward and reverse rotation of a conveyor belt is not simply a change in direction of rotation, but a mechanical challenge involving a reversal of the tension distribution
• If a one-way (head-driven) conveyor is easily reversed, the carrier side will loosen, causing slipping and snaking.
• When reversing, the "slack side tension" in Euler's formula tends to be insufficient, and the effective tension must be carefully reduced.
• Initial tension control is important to prevent slippage, but excessive tension shortens bearing life, so a solution should be found in the drive system
• For bi-directional conveying, the "intermediate drive (center drive) method" is most suitable because the tension distribution is nearly symmetrical in forward and reverse rotation.
• Head drive is prone to instability in reverse and should be limited to light-load, low-frequency applications
• For forward and reverse rotation, it is essential to use a "belt with V-guide" that physically restrains lateral displacement.
• Meandering control using only crown pulleys is risky because it cannot cope with sudden changes in behavior when reversing.
• Check the parallelism (diagonal dimension) of the frame and observe the minimum pulley diameter to prevent accidents caused by riding up the V-guide.
• Note that reversible motors are rated for 30 minutes and are not suitable for automatic machines that run continuously.
• To achieve both continuous operation and controllability, a combination of "three-phase induction motor" and "inverter" is recommended
• Inverter acceleration/deceleration control (soft start/stop) can mitigate workpiece tipping and belt impact
• Frequent reversals and sudden deceleration require installation of "regenerative resistors" for regenerative energy handling and calculation of moment of inertia
• Reverse operation creates new "entrapment hazard areas" at the end of conveyors, etc., so safety covers and other countermeasures should be provided.
• Calculations based on JIS standards and selection based on appropriate safety factors ensure trouble-free, highly reliable equipment.

That's it.