Mechanics of Vibratory Equipment
The system I am currently working on is a machine that packages small plastic components. It consists of a number of hoppers with linear vibratory feeders that convey the parts into a mechanism that weighs the parts and feeds them into a vertical flow-style bagger. The vibratory section of this system was manufactured by RNA Automation. The picture above shows a system on their website feeding metal parts; the system I am involved with is quite a bit larger.
In a previous post I discussed some of the mechanics associated with vibratory bowls. Since the machine I am working on uses linear trays I will share some of what I have learned.
Linear vibratory systems use similar controllers to those used in bowls, however they may be larger (i.e. higher horsepower) due to the larger mass that must be moved. Historically, many vibratory systems used electromechanical means to vibrate a structure. This might consist of a motor with an off-center weight attached to the shaft. Most vibratory systems now use electromagnetic coils to generate the vibrations, but the controlling principle is the same.
The mechanical system itself is generally tuned to a specific frequency. In the US this is usually 60 Hz, but in Europe most of the manufacturers use 50 Hz. Since variable frequency drives are typically used to generate the vibrating impulses there is no specific reason to use one frequency over the other except for convenience. Some systems are tuned for double this frequency, especially smaller ones. Thes are known as full wave systems and are generally tuned at either 100 or 120 Hz. 50 and 60 hz systems are half-wave systems.
Tuning of the mechanical system involves using weights and springs of a specific size and positioned at locations that make use of a resonant frequency. Changing the positions and spring constants can alter the resonant frequency slightly and it can be a bit of a “black art”.
After the mechanical part of the system has been designed the variable frequency drives can be used to further tune the system. Since mechanical tuning of the drive can be imprecise, tuning the drive to the system can optimize the system for maximum effect. The two parameters that can be adjusted on the drive are the frequency of the output waveform and the amplitude of the output. The way to determine the optimum frequency for the system is to start at the designed resonant frequency of the system. Typically the frequency setting on the drive will be incremented down one-tenth Hz at a time until maximum movement is achieved. Usually lowering the frequency increases the speed; occasionally if decrementing the frequency downward doesn’t increase movement the frequency will need to be increased. It is generally not advisable to try and adjust the system mechanically.
When the maximum vibration has been attained by tuning the frequency, amplitude can be set to achieve the proper amount of movement. On the system I am currently working on the amplitude is controlled using a 0-10v signal. Since the system runs many different sizes of product, a recipe of amplitudes and on-off pulses are used to achieve the desired product flow characteristics. There are five different linear tracks on this system and they all work together to get the right flow of parts. This system is designed to package exactly the right number of parts, so one of the tracks is optimized to feed one part at a time into the scale assembly while allowing proper settling time for the scale reading. This is done without means of a gating or escapement mechanism since they could potentially damage the fragile plastic parts.
An additional method used to increase movement of the parts in the desired direction is the use of directional carpet. This is a synthetic carpet where fibers are oriented in one direction to achieve thrust. Many linear feeder manufacturers use and sell this material, which is made by companies like 3M. Since the carpet wears it occasionally needs to be replaced.
Vibratory equipment is used in part feeding, food service and bulk handling applications. It is often considered a type of conveyor in material handling of bulk product since there are no crevices for product to build up in. Most vibratory pans, trays and hoppers are made of stainless steel. Some may also be lined with teflon or other plastics to reduce damage to parts.