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mortenhuse

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Reply with quote  #1 
Hi all vibration experts!

I have a challenge of both practical and theoretical dimensions. We mount jet fans for tunnel ventilation to a suspension frame (which in it self is connected with a stiff connection to rock bolts to the tunnel ceiling, see 1st attachment for sketch). Our jet fans have 4-pole motors hence a rotation speed of approx 1500 rpm and a natural frequency (1X) of approx 25 Hz. This is the larges contribution to vibration and which is my focus for the dampers. 

My problem is that the manufacturer of the bolt gives recommended tightening torque for a given bolt (e.g. 170 Nm for M16 bolt grade 8.8 stainless steel), which results in a axial force/clamping torque which is much higher than statical load limit of the damper materials that have damping properties at 25 Hz. What do you experienced vibration experts do in these cases?

I have chosen the vibration damper (damper A) material to fit the weight of the load and still meet the required damping property at 25 Hz, however I do not know how to both ensure a proper clamping torque bolt/nut and ensure good vibration damping? And I assume that the degree of clamping torque will effect the properties of the two dampers. What is the best way to do this properly? Please help!

PS: we do have some practical limitations to the dimensions of the dampers, hence total damper height is approx 25 + 12,5 mm and width-length is max 80x80 mm.

Regards Morten

 
Attached Files
pdf Vibration dampers setup jet fan to frame.pdf (197.38 KB, 42 views)
pdf Material Data Sheet Sylodyn ND EN.pdf (152.62 KB, 23 views)
pdf Material Data Sheet Sylodyn NE EN.pdf (153.72 KB, 11 views)
pdf Material Data Sheet Sylodyn NF EN.pdf (151.30 KB, 9 views)
pdf FreqCalc_Sylodyn_NE-25mm (1).pdf (451.76 KB, 19 views)

John from PA

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Reply with quote  #2 
Well, without going to in-depth in the details you've provided, the obvious question (to me) is can you change the bolts to something that doesn't exacerbate the issue? 
Dan Timberlake

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Reply with quote  #3 
I think you need a steel tubular spacer to allow torquing the bolts and not compress the isolation pad material.  And rubber between the OD of the steel spacer and the frame hole ID.

As shown on page 5 here - 
http://rubberpartscatalog.com/sites/default/files/docs/Rubber-Parts-Catalog-Delta-Flex-Vibration-Control-LORD-CENTER-BONDED-MOUNTS.pdf

============================

The calculations are for an infinitely stiff support frame.  Due to real world support frame flexibility, Many times the isolation mount must be significantly softer ( greater deflection without bottoming) than calculated to avoid creating a dual mass problem.
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RustyCas

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Reply with quote  #4 
1X vibration is nearly always the largest component. The correct solution is to balance the fan well (insitu preferred) and keep it clean.

Tightening torque serves 3 purposes: to keep the fastener in pure tension, to "prove" the strength of the bolt (if it doesn't fail when torqued, then it should never fail), and to connect the mating surfaces with enough force so that lateral forces will never cause the surfaces to "slip" (i.e., to keep everything "tight").

So tightening torque is irrelevant using an elastic damper. You have to use a bolt with a locking nut, and then some other means of preventing lateral (or axial) movement, such as additional "snubbers" or a bushing that allows vertical movement, but not horizontal. (This is what Dan is referring to (I think).

The other method would be to use a damper where each side is bolted to a frame, without a bolt crossing the "joint."

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mortenhuse

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Reply with quote  #5 
Hi all,

Thank you for your insightful replies. Balancing, service and maintenance of the fan is a high priority. I understand that my setup is far from ideal, so new solution are very welcome.
Dan: do you have some more info (another link or doc) which describes the E-modulus at given Freq. and static loads etc., hence the damping properties (and natural freq) at given frequencies for these materials, as shown for the example material in the attachment? It has been difficult finding "rubber-like" materials that have a proven damping at 25 Hz +-5. The only material I have found is Sylodyn (https://www.getzner.com/en/products/sylodyn) and solutions including springs are not an option for the customer.
Do you think the producer, Data-Flex, can provide info on the vibration damping at 25 Hz given some dimensions and lodes for the product you referred to on page 5 in the borcure?

Thanks again!

Regards Morten

 
Attached Files
pdf Material Data Sheet Sylodyn NE EN.pdf (153.72 KB, 10 views)

Ron Brook

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Reply with quote  #6 
Danny is correct. You dont torque down on the absorbing material use a tube spacer to hold everything in place and then torque the bolt down to it.
Beatnik

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Reply with quote  #7 
Quote:
Originally Posted by mortenhuse
Hi all,

Thank you for your insightful replies. Balancing, service and maintenance of the fan is a high priority. I understand that my setup is far from ideal, so new solution are very welcome.
Dan: do you have some more info (another link or doc) which describes the E-modulus at given Freq. and static loads etc., hence the damping properties (and natural freq) at given frequencies for these materials, as shown for the example material in the attachment? It has been difficult finding "rubber-like" materials that have a proven damping at 25 Hz +-5. The only material I have found is Sylodyn (https://www.getzner.com/en/products/sylodyn) and solutions including springs are not an option for the customer.
Do you think the producer, Data-Flex, can provide info on the vibration damping at 25 Hz given some dimensions and lodes for the product you referred to on page 5 in the borcure?

Thanks again!

Regards Morten



If I understand correctly it's not dampers that you are installing, it's isolators. It's not made to reduce the vibration of the fan, it's made to prevent the vibration to go in the structure and floor. 

You could explore other options, specially if there is possibilities that the impaller become dirty. With isolators you might still having a machine operating at natural frequency. Increasing/decreasing stiffness, adding/removing mass or installing a tuned absorber could work. 

Your isolators will be decreasing the stiffness of the assembly, so you might be lucky and move the natural frequency enough to reduce the vibration.

Are you sure the natural frequency is directly at 25HZ? you will move the natural frequency down with isolators, so I'd be more confident in your success if your tests indicate that the phase shift happen around 23Hz and not 27Hz...



Curran919

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Reply with quote  #8 
Quote:
Originally Posted by Beatnik



If I understand correctly it's not dampers that you are installing, it's isolators. It's not made to reduce the vibration of the fan, it's made to prevent the vibration to go in the structure and floor. 

You could explore other options, specially if there is possibilities that the impaller become dirty. With isolators you might still having a machine operating at natural frequency. Increasing/decreasing stiffness, adding/removing mass or installing a tuned absorber could work. 

Your isolators will be decreasing the stiffness of the assembly, so you might be lucky and move the natural frequency enough to reduce the vibration.

Are you sure the natural frequency is directly at 25HZ? you will move the natural frequency down with isolators, so I'd be more confident in your success if your tests indicate that the phase shift happen around 23Hz and not 27Hz...


I'm glad somebody was interpreting this the same way I was. I think OP needs to better define what exactly they are trying to accomplish, because there are some red flags that maybe OP is not going in the direction that they should be to solve their problem. with a spacer and bolt running parallel to an 'isolator', the element essentially does nothing. all the vibration will be transmitted through the elements that offer the most stiffness, i.e. the spacer and bolt. Having an unloaded rubber element (significantly less force than the other load path) will be inconsequential.

And these spacers... in parallel to compliant layers or isolators, this is a problem that I recently ran into with a clusterfuck of a warranty call on one of our pumps with disappointingly shoddy assumptions on the load paths of the foundation. The contribution of flexibility that compliant elements add to a 2-3 load path joint is only proportional to the amount of load they take!

Even in the centerbonded mounts that Dan posted, the tensile bolt is not normally engaged (or not loaded significantly during usage compared to the compressive load of the compliant element).

Whether OP is trying to isolate or detune resonance (is still unclear to me), it seems that they are not going to achieve either.
mortenhuse

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Reply with quote  #9 
Hi all,

Thanks again for the valuable input. It is true, Beatnik, that I have not been very clear in describing my goal for the setup, and you are right. The true goal of the material is in the function as isolators and not as dampers. Hence preventing vibrations to propagate into the suspension frame and other structural materials adjacent or in worst case end up with resonance problems due to same vibration frequencies of jet fan and frame natural frequencies.

The fan it self has fairly high requirements for balancing (G2.5) and normally measured in all three directions on fan casing < 1 mm/s (rms) O/All. So as long as the jet fan is kept well balanced, the only thing I am concerned about is resonance with eigenfrequencies of the suspension-frame-rock bolt setup.

From bump tests, the spectrum shows a lot of eigenfrequencies from the frame-rock-bolt-assembly. The main vibration frequencies from this fan is always 24,7 Hz (1X of 1481 rpm) and 6X (1X times number of blades) and will always be there to some extent. These are the main frequencies we try to stop from propagating to the frame, regardless of the design of the frame.

Last year we ended up in an unfortunate situation where the fan isolated measured approx. 1,5 mm/s (rms) O/all, but when fixed to a suspension frame of certain dimensions and design, the vibrations was measured to >10 mm/s (in only vertical direction or radial vertical). This was "solved"/reduced after both increasing the vertical stiffness of the frame and installing isolators between the frame and jet fan to "change the mass" or stiffness of the whole assembly, in order to change the eigenfrequencies of the "frame" away from 24,7 Hz, which is the natural frequency (1X) of the 1481 rpm rotating motor and the peak that had the high amplitude. However we are not 100% satisfied with the result and the "science" of the solution. We are always having to compromise between having isolators with the "proper" load/torque resulting in good isolation for > 20 Hz and the desire of tightening a bolt/nut to ensure that everything is and will be in place over the thousands of cars that are driving beneath the fans.

I agree with Curran919  that using the "spacer and bolt running parallel to an 'isolator'" most probably will not avoid vibration from the fan to propagate to the frame, and in worst case in resonance with eigenfrequencies of the frame, hence not solving our problem.

Unfortunately we have not found a deign/setup that will ensure that we never end up with increased vibrations due to resonance. Hence due to different requirements and dimentions in different projects we always have a small risk of ending up with a total system that has vibration levels higher than we accept. Any thoughts are more than welcome!

Regards

Morten
Curran919

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Reply with quote  #10 
Hi Morten,

Thanks for the re-explanation. It is much clearer where the problem stands now. Allow me to reiterate to ensure we are on the right page. The idea (as usual) is that you simply want vibration on the fan bearing (or any part of the fan, really) to not exceed about 1.5 mm/s. Any foreseeable highs would likely be a result of resonance. You can not change the excitation frequencies (fan speed is constant?), so in case their is resonance, you must detune the resonance by changing the offending natural frequency.

The structural natural frequencies of your machine/frame are based almost entirely on just mass and stiffness. Generally, changing the mass is only for desperate/temporary situations, so the tune your natural frequencies, you want to strategically add or remove stiffness. For multiple identical units, the natural frequencies should be pretty constant. Its when you start adding in 'soft' or non-linear components like isolators and the like that the variance between machines increases, which is generally undesired, and there can be significantly more creep, which is definitely undesired.

If you are past the design phase, then the most straight-forward choice for detuning is often to 'throw steel at it' and stiffen up the structure with additional beams/gussets. It is important to remember that the fan and frame are all one system and they must be considered as such when starting to mess with the natural frequencies.

If you find that the problem pops up randomly on some of your machines, the first thing I would do is identify where the variance originates and fix it. If you just have to accept the variance, than you will have to perform modal analysis on every fan to identify whether a resonance needs to be detuned and act accordingly. One option is to design in a variable stiffness element in to the frame or mount. This can involve putting different "isolators" into a load bearing path, but then the "isolator" must be designed to carry the majority of load at that point (no parallel bolt/spacer). If you are using soft elements that are actually advertised as isolators, it is likely that they will be much softer than what you would want in this application (as a first guess). A stiffer, stronger element may be a good option without knowing much about your project.
RustyCas

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Reply with quote  #11 
Why don't you just use spring isolaters? I'm really not up on the engineering of these (others here will be) but I've seen them on a variety of machines, large and small, and they work well. I've taken readings on either side of the isolator, and vibration just does not transmit through them.
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OLi

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Reply with quote  #12 
As a rule of thumb the aim is to have the mounting resonance 10x lower than the vibration freq. so in this case aim is 2.5Hz it may be adjusted due to limitations. That goes for all 6 directions...... Rubber or steel is maybe a cost question, steel may have lower damping and may need to be filled with steelwool or have a damper.


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