At the turn of the century, automobiles may have felt like a miracle, but the teeth-rattling, hip-displacing reality of riding one was anything but miraculous. Many early machines wore out fast or were replaced entirely because they lacked the technology to control vibration or absorb shock. Complex yet simple solutions are now in place that can protect machinery and the people who operate it.
The smart machines of the modern age aren’t just vehicles of conveyance, they are transforming how we live. Everything from manufacturing to roboticized equipment benefits when unwanted movement is minimized. The parts that accomplish this are typically made from rubber or metal and expertly manufactured for optimal use.
At RPM, vibration control and shock absorption, often referred to as Noise, Vibration and Harshness (NVH) mitigation are part of our standard and custom offerings. There are numerous materials and forms these materials take. Every coupling, mount, isolator, or bushing we make is perfectly engineered for optimal use. Read on for a comparison of the common materials and best options for shock and vibration control.
Read on to learn about:
Elastomers v. Metal Springs
Moving parts aren’t a back and forth operation. Machine parts and attached components move in multiple directions and on various planes. Even minor misalignments can seriously impair machine function and increase wear and fatigue on components. Two frequently compared options for maintaining the integrity and minimizing unwanted movement are elastomers versus metal springs.
There are a few criteria with which to compare these two options.
Machinery is subject to both intermittent shocks and continuous vibration, this often is detrimental to the machine longevity and function. Vibration issues are transmitted downstream. Flexibility is preferred over stiffness, however too much flexibility or motion can create new problems - we strive to find a balance between the two. Most people are familiar with standard coil spring. You apply a force, and the spring compresses by some distance. This is called the spring rate.
Surface motion, over time, can cause damage to this design. Coil springs, while good in many applications, have one drawback - that is, they don’t dissipate energy quickly. Meaning the input can resonate for quite some time. The spring rate is often linear.
One of the benefits of elastomers over metal springs is that they dissipate (absorb) energy, and the spring rate is variable to some degree. Meaning the elastomer is better suited to absorb energy from the system without transferring it as a metal spring system would.
Bump compliance is often measured through bump tests, learning transfer functions, intertance, dynamic compliance, or mobility. Knowing the natural frequency and exciting frequency (which results in resonance), damping measures can be correctly applied. The margin or resonance varies from machine to machine. Most commonly, rubber and rubber elastomers outpace coiled metal springs for damping bumps.
The fatigue cycle of elastomers versus metal should be considered. It’s the one area in which metal may have the advantage. Elastomeric polymers do have high thermal resistance, which means they can withstand high heat caused by friction. However, their degradation rate is higher than in many kinds of metal. This may mean that they need to be changed out more often to continue the desired function.
1. I Need to Select a Vibration Isolator For My Application: Where Do I Start?
For any new project, choosing a vibration isolator is a daunting task. Part options seem limitless, and the specifications are rarely easy to understand. Often, suppliers will refer you to their catalogs and let you handle the rest. That leads to suboptimal choices, returns, and even catastrophic failures. Simply put - choosing the wrong isolator can amplify the input rather than isolate - causing more problems than it is solving.
And that is no surprise. Key points in deciding the correct part are complex. Vibration isolator buyers need to review specs like these:
- Weight limits: the weight that individual isolators can support and still work effectively
- Vibration dampening: the process of dissipating vibration into heat, which varies by material
- Natural frequency: the frequency a component or system naturally vibrates
- Transmissibility: the ratio of the vibration entering a system, to the vibration leaving it
- Temperature sensitivity: how the ambient temperature impacts individual parts
And there are many more application-specific criteria to consider. As such, selecting the incorrect part happens frequently.
What happens when I choose the wrong option?
There can be severe consequences for choosing an incorrect vibration isolator. For example, dealing with natural frequencies, think of vibration as frequencies and vibration isolation as impedance types. The material you choose works best at specific input frequencies. That also means that while a material dampens vibrations in one range, it can amplify them in another. Understanding the system and the frequencies becomes more important to successful system isolation.
In a lab setting with small 3600RPM centrifuges and mixing units, neoprene mounts may be perfect. If you then take those same mounts and try to use them for the 900RPM exhaust fans in the next room, you may find they amplify the vibration. This amplification would increase noise, but also may cause enough vibration that microelectronics in the sensitive medical equipment you have are now unusable.
How does RPM ensure that you’re getting the correct part?
RPM handles part selection differently than other rubber manufacturers. We want to see people succeed. Taking time to understand your needs and find the perfect part for you is vital to our business. And if we find out that the ideal position doesn’t exist, we can make it.
Before our engineers can offer recommendations, we need to know about your project. They will need a base understanding of the problem you need to solve and the environment you are working in.
We start that process by asking six questions:
1. Is the component stationary or mobile?
Stationary and mobile components take two very different approaches. Stationary components require a simple mounting system where you sandwich a vibration isolating material between two metal pieces. This simple solution can offer more accurate machining, longer product life, and reduced maintenance and is cost-effective.
Mobile devices require more specific and often complex solutions to make sure they are safe. Mobile equipment, particularly off-highway vehicles and marine applications, for example, experience more vibration than their stationary counterparts. Now, what if that rubber were to fail? Would your unit remain intact or separate from your system? Selecting a mount that is “safetied” becomes a critical consideration. You also need to factor in the varied external conditions they will experience. Numerous mounts can accommodate these, but they will need to be carefully selected to meet user-specific criteria perfectly.
2. What is the weight of the component we are isolating?
The weight of the component will directly impact what materials can be used to isolate them. While some materials may work well with your vibration. Static and dynamic loading on the isolation system need to be considered. Coupled with the inertial forces can the isolators contend with this? Pound-force per square inch (PSI) is a common failure point for incorrect materials.
A qualified engineer from RPM can provide insight into weight issues. Contact us for expert guidance.
3. How many mounting points will you have?
The mounting points are important and related both to weight and motion. The weight of the piece compared to the number of mounting points inform the minimum material strength. The number of mounting points can also lead to substantial differences in design.
4. Center of gravity details
For an effective vibration isolator, we must know where the center of gravity is. We also need to know where it is in relation to our isolators. We need this to get an accurate picture of the system dynamics, it becomes a very important isolator design consideration Again, this information is tied to understanding the loads and potential loads at the mounting points. Offset centers of gravity can cause very different loads for individual isolators.
5. What is the vibration source we are isolating?
Fans, motors, and pumps require different approaches to vibration isolation. Because each has variable frequencies and maximum allowable motion, each will require specific types of materials and mounting systems.
6. What are the environmental conditions?
Different elastomers have different responses to the environments in which they operate.. All of these questions are vital to know which materials will respond best for your application.
2. Does the Size and Shape of a Vibration Mount Matter?
If it shakes, rattles or rolls, chances are you can solve vibration issues with rubber. Custom manufactured vibration mounts are commonly used in machinery. This is because they relieve vibration, which can pose operational challenges and deteriorate equipment. Vibration and shock isolation isn’t a simple issue, however.
The complexity of solving noise, vibration, and harshness (NVH) issues requires thoughtful, often customized, solutions. Magnitude, pulse shape, orientation, and load direction are all part of this conversation. The size and shape of vibration mounts are essential to proper function. Here’s why:
- The size of a vibration mount or any rubber pieces is important because the less rubber, the less load you can put on it. Larger sized elastomers can accommodate more load but introduce more motion. There should always be an accurate measuring process to ensure that the size not only fits but has both the flexibility and durability to not require frequent replacement.
- The shape of a vibration mount matters because the rubber is under compression. It needs to dampen or absorb shock from motion across multiple planes and directions. This may require a custom design or shape.
Size and shape matter. What vibration mounts are made of matters, too. Your best options will probably be natural rubber or neoprene.
Application for the Vibration Mounts
Vibration isolation mounts come in a few basic shapes and designs. They are essentially supporting structures that absorb impact, removing force and shock from equipment that would otherwise break down faster or incur damage.
Types of Vibration Mount
There are a few basic kinds of isolating mounts that provide bracing on different machine parts:
- Two-piece vibration mounts
- Anti-vibration pads
- Small industrial engine mounts
- Plateform mounts
- Rubber flex bobbin sandwich mounts
- HVAC neoprene mounts
- Large sandwich mounts
- Conical mounts
- Grommet isolators
- DynaflexⓇ couplings
- Machinery mounts
- Flex-bolt sandwich mounts
- Binocular engine mounts
Read more about types of sandwich mounts here.
Vibration Isolation & Isolating Frequency
Understanding both the movement and the frequency of your vibration is a critical part of selecting the right mount. Parker LORD, a supplier of our vibration isolators has an in-depth article on the theory of vibration isolation.
If you’re unsure of the right mount type, number, or placement, or even the material selection, give us a call. We have engineers available to help you identify the right fit for your application.
3. What Role Does Durometer Play in Selecting a Vibration Mount?
Durometer is perhaps the most standard, go-to test for rubber parts. It tests the hardness of a piece. While it may seem official and straightforward, the engineers at RPM find that it isn’t actually giving clients the right insight.
The test itself has issues and there are plenty of incompatibilities that can pass unnoticed. If this occurs—and a part in question is installed and in use—it may wear out faster or not be up to the intended task. To avoid this, and learn what additional metrics to look out for, read on.
4. Is Durometer the Correct Test?
A durometer gauge comes with an indenter. This essentially measures the hardness of a rubber part. When used in isolation, a durometer test is insufficient to truly understand the nature of the piece and whether it is right for a specific application.
Here are two reasons a durometer test may not give you the full picture.
- Durometers may identify problems on the surface of a part. The problem is, surface deformities aren’t the only issues that could foretell issues.
- ASTM D2240 stipulates durometer readings from flat, parallel surfaces. Rounded, rough, or uneven surfaces cannot be adequately measured by durometer gauges.
Additional issues could include an inability to get a proper reading. For instance, many rubber parts are bonded to metal or don’t have flat surfaces. To get the gauge perpendicular to the surface is virtually impossible on a rounded surface or one with more than one kind of material.
Durometer is usually a key feature cited by manufacturers. But when it comes to selecting a vibration mount, it isn’t the only facet and may not even be the most relevant one. At RPM, we are intent on providing the right parts to the right people. This may mean a deeper dive than traditional ratings or feature lists.
Another issue is that parts can have the same durometer rating and be completely different parts. They may have different loads and be completely different sizes. To buy something solely based on the durometer result could yield a part that’s the wrong fit for the desired application.
Regarding vibration mounts or other stabilizing rubber pieces, there are a few different aspects every buyer should understand. Let’s review what those are.
Load and Deflection
The load deforms a rubber part. Testing force stress and compression are vital to understanding how a part will perform in operation. These tests should be performed on rubber in shear or compression. There are relevant outcomes for testing both static and dynamic load deflection.
- Static load-deflection specifications may supersede hardness. The part should be tested for static load when at rest, typically in shear or compression.
- Dynamic load-deflection specifications relate to vibration-isolating applications. Rubber may increase in stiffness when it is dynamic.
Dynamic testing measures things like steady-state resonance, steady-state non-resonance, and rebound evaluation. Both static load and dynamic load may be a part of spring rate tests.
Spring Rate Tests
Loading a part in compression or shear can determine the spring rate. It’s also an indicator of a stress-strain relationship, which is the core quality being tested in all of these scenarios. Once a part is subject to spring rate testing, it will become clear whether it is the right fit, shape, and material for the intended application.
The size and shape of a part influence how much a part will compress or retract. Spring rate takes the resiliency or rebound of a part into account (a durometer does not). Once you have this rating, you’ll know the proper response of a part.
These tests should be conducted by qualified professionals who can interpret the results, most commonly in lbs/in (pounds per inch) or N/mm (newtons per millimeter). Numerous factors can influence the outcome, including dynamic history, age test conditions, and even temperature. The benefit of expertly conducted tests is a spring rate measurement, which is superior to a durometer in both relevance and accuracy.
Parts that can be tested for spring rate include things such as:
- Sandwich mounts
- Compressor mounts
- Industrial engine mounts
- Vibration isolator mounts
These parts will serve numerous purposes in machines, including isolating vibration and damping effects. RPM provides compression-spring rate, compression static load, shear-spring rate, and shear static load ratings for most of our manufactured rubber parts.
Buy the Correct Rubber Parts
At RPM, we supply both standard and custom industrial rubber parts. We specialize in facilitating a client-centric experience that ensures you are buying the right part. Our team of highly skilled engineers can provide expert insight and guidance.
Our catalog of manufactured rubber parts includes molded rubber, vibration control, extrusions, and tubing and custom rubber parts.