Vibration can be the enemy of a plant manager’s best-laid plans. For example, dosing machines measure out a precise amount of product, in liquid or powder form, from a large supply according to a predetermined level or volume or weight into a container. For plant managers, a great deal of time and research goes not only into keeping the machines running at optimum throughput, but also selecting the correct machine in the first place. Vibration from surrounding equipment can undermine maintenance and the time spent carefully selecting precision equipment.

In linear motion systems, vibration control products operate by decoupling the equipment from the vibration source, making it difficult for the vibration to make its way to the equipment. For example, vibrating tables are often used to transfer components and materials in many manufacturing processes, including linear motion systems.

The frequency of the vibration causes the material being handled to subtly bounce around and flow across the vibration surface. Although this is an effective method of moving material and components in high volume production, the same vibration that allows for this movement can also damage the surrounding equipment. By isolating the vibrating assembly with wire rope isolators, the surrounding equipment is protected. Isolating the surrounding equipment from the damaging vibration prevents damage to bolts and welds.

In addition to controlling vibration within a linear system itself, vibration control devices are also used to steady a system’s electronic components (the brains behind the operation), as they are typically expensive to fix or replace and can cause significant downtime if they are damaged.

Prescribing Vibration Isolation

A pharmaceutical maker realized that vibration was affecting the performance of its dosing machines at one of its factories. The source of vibration was coming through the factory floor from surrounding equipment. Over time, the dosing machines began operating incorrectly because of the situation. The precision equipment calibrating the doses was affected because of the ongoing vibration. Even the solder points inside the machines’ electrical panels were breaking due to the effects of the vibration frequency.

Wire Rope Isolators.

To resolve the problem, ITT Control Technologies tuned and put in place four of its WR Series (WR12) wire rope isolators for each dosing machine. These isolators weigh about 1.2 kilograms each and consist of stainless steel braided cable, wound between two sets of aircraftgrade aluminium mount bars. They are 215 mm long, 76 mm tall and about 100 mm wide, and function to prevent both shock and vibration.

When the wire rope isolators’ strands flex from vibration input or shock impulse, they convert kinetic energy into thermal energy, which then dissipates to the atmosphere. ITT’s engineers tuned each system of the wire rope isolators to isolate the dosing machines from the dominant floor input frequencies of 15 Hz and 24 Hz. These 15 Hz and 24 Hz inputs were being transmitted through the floor from the surrounding equipment in the factory.

The challenge for anyone installing an isolation system is to size the isolator based on the dynamic performance of the isolator as well as the weight it is supporting (in this case the dosing machine) in order to ensure its resonant frequency (the frequency at which the sprung system will amplify a vibration input) is sufficiently lower than the source of the input frequency (the vibration stemming from the neighbouring equipment). The pharmaceutical maker measured the vibration to assist ITT with its calculations. But, in other cases, ITT conducts the measurement, or the frequency is estimated. For instance, it is reasonable for an engineer to know a generator turning at 1,800 rpm will translate into approximately 30 Hz.

By tuning the wire rope isolators to have a resonant frequency of 7 Hz, ITT was able to isolate approximately 70 percent of the 15 Hz input, and 90 percent of the 24 Hz vibration input. It is critical to size the isolators for the resonant frequency of the system.

In most cases, when a maintenance department decides to use wire rope isolators, plant managers will study a catalogue to find a wire rope isolator rated to support the weight of their machine. But it is important to pick and tune the isolator based on the dynamic performance of the isolator, not just the weight the wire rope isolator will carry. An incorrectly sized isolation system has the potential of having no effect, or in many cases making the vibration worse.

By relying on the dynamic analysis performed by ITT, maintenance managers can predict how much vibration isolation they will achieve. The increased deflection capability of the wire rope isolator enables the product to not only isolate vibration, but also withstand shock and protect equipment, allowing a deflection of up to 40 mm in the pharmaceutical maker’s dosing machine application under a high-force shock impulse.

Heavy industry Shock Absorbers from ITT.

Not Only Vibration, but Shock

Along with vibration, shock is detrimental to maintaining a machine for peak performance. In extreme situations, shock can damage equipment and injure workers. One instance where shock (if an energy absorption device is not considered) can cause damage is with overhead crane systems.

These systems are often mounted to girders above the factory floor. If an operator loses focus or a motor on the crane’s trolley malfunctions, a runaway condition can send a trolley and load crashing toward the end of the crane bridge. Shock absorbers prevent damage or injury when a high-shock situation happens. It’s not uncommon to see factory managers rely on elastomers, or even blocks of wood, to absorb shock.

These alternative methods do not have the energy absorption capacity or stroke length to achieve any significant reduction in shock force transmitted to the equipment. Earlier this year, a factory manager called on ITT engineers to install ITT Heavy Industry (HI Series) shock absorbers for a 150-tonne overhead crane application. The shock absorbers protect heavy machinery and cranes during the transfer of materials as well as in the case of a runaway crane trolley. The HI Series shock absorbers range in size from 50 mm bore x 50 mm stroke to 210 mm bore x 1,000 mm stroke. ITT installed two shocks at the end of the crane’s rails, and chose the size based on the weight of the crane along with the maximum velocity the crane trolley could travel (100 m/min) in a runaway condition. The trolleys, too, have a set of ITT shock absorbers.

It is important to consider more than just the weight of the material-handling device when considering shock protection, whether it is an overhead crane, rail car, assembly line, or overhead conveyor system. The velocity of the impact plays an important role in the amount of damage that could occur in the event of an impact, as it has a squared function in the calculation of kinetic energy (E = ½mV2). By fully analyzing the shock event, ITT can insure that equipment is protected.

Calculations of weight and velocity will help a machine builder or maintenance engineer determine the type of high-performance shock absorber to use. Analysis is also important for worker safety. According to a report from the European Agency for Safety and Health at Work titled Workplace exposure to vibration in Europe: an expert review the writers state.

– The only effective way to deal with the risks related to vibration is to set up a strategy based on assessment and evaluation. The basis for this strategy can be found in the legislation, whereby the directive obliges employers to assess and, if necessary, measure the levels of mechanical vibration to which workers are exposed.

When looking for a way to minimize damage and increase meantime between failures in your linear motion application, it is important to consider utilizing vibration and energy control devices. Tuning your solution to match unique application requirements can often lead to improved results, including: protecting equipment, increasing product life, improving plant efficiency, reducing plant downtime, lowering maintenance costs and ensuring safety.

Chris Kudla,

senior application engineer, ITT Control Technologies