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  1. EnglishMuffin

    trick question on beams

    According to my copy of Blevins, Hacksaw is correct (ie identical natural frequencies), but the mode shapes appear to be somewhat different.
  2. EnglishMuffin

    vibration on bearing cage

    I should probably clarify that the type of computer programs I am talking about do not necessarily have much to do with FEA, at least not as normally understood, although they do arrive at a result by solving a large number of simultaneous non-linear equations - along the lines first pioneered...
  3. EnglishMuffin

    vibration on bearing cage

    Well, most programs are proprietory (RHP, SKF, FAG etc) - the first thing to try is a bearing manufacturer. Some of them have developed very sophisticated cage analysis software. I used to use a program called ABODE written by a non-bearing manufacturer - that would calculate excursion, but the...
  4. EnglishMuffin

    vibration on bearing cage

    Are you talking balls or rollers here, and what is the cage made of ? One thing that can cause cage failure in a ball bearing, particularly with phenolic cages, is excessive ball excursion (ie tangiential oscillatory ball motion that exceeds the clearance between rolling element and cage). Ball...
  5. EnglishMuffin

    Effect of prestress on torsional vibration?

    Hmmm - 4 replies to my question. At last I have time to reply - I have been so busy lately. Greg Locock mentions four effects, all of which I think I considered when I did this experiment almost 20 years ago. I think I gave all the relevant data, except for the bulk modulus of the oil - I have...
  6. EnglishMuffin

    Effect of prestress on torsional vibration?

    This discussion brings to mind an experiment I once did, although it's largely irrelevant. I was having a discussion with a fellow engineer about axially pre-tensioning ball screws to increase their natural frequency (which from the standpoint of "whirling" is an important consideration...
  7. EnglishMuffin

    Effect of prestress on torsional vibration?

    suviuuno : My response does NOT explain why torsional natural frequency is related to stress. It does not even intimate that bending natural frequency is related to stress - only that it might erroneously appear to be related because of an incidental correlation. For the sake of argument, it...
  8. EnglishMuffin

    Effect of prestress on torsional vibration?

    So called "Gyroscopic stiffening" occurs with large diameter heavy rotors on flexible shafts - the higher the speed the greater will be the resistance to local beam rotation at the rotor connection points, and hence the bending natural frequency will increase with speed. It is not caused by...
  9. EnglishMuffin

    Oil filled vibration damper

    Actually, this is a large machine tool, with a 500mm by 630mm ram, about 12 feet long, containing the machining spindle. The problem is to optimize the dynamic stiffness of the ram (ie - it is not an isolation problem). The previous version of this did indeed utilize hydrostatic bearings, but...
  10. EnglishMuffin

    Oil filled vibration damper

    Well, the (anti) friction pads (Rulon) are necessary (I think) because the component being damped has to move transversely relative to the pads. They would be lubricated of course. I can't see any easy way to do it without some sort of seals - I had envisaged two small stub shafts with seals on...
  11. EnglishMuffin

    Oil filled vibration damper

    I guess I am designing what amounts to a dashpot - ie a piston inside a cylinder with clearance. The piston will be connected to a friction pad pressing against a sliding surface, which is independantly supported by rolling element bearings - which provide most of the stiffness. There will...
  12. EnglishMuffin

    Oil filled vibration damper

    What is the best way of designing an oil filled vibration damper ? The frequency range of concern is perhaps 30-80Hz, at low amplitude (0-.001"). Should such a damper utilize a relatively long viscous restriction or is a short orifice good enough ? According to my edition of the Vibration...
  13. EnglishMuffin

    Designing a dynamic vibration absorber

    Yes - that makes sense - it just wasn't clear to me whether you were actually just talking about "undertuning" the frequency of the auxiliary mass in isolation, which you have to do regardless of anything else with a damped absorber even to get two equal amplitude peaks. In my experience you...
  14. EnglishMuffin

    Designing a dynamic vibration absorber

    It's not quite clear to me what Greg is saying either. When you add a vibration absorber to a structure with a pronounced natural frequency, whether damped or undamped, you create two natural frequencies where before there was only one. And the textbook approach in the case of a damped absorber...
  15. EnglishMuffin

    Designing a dynamic vibration absorber

    Just as a point of interest, there is a sort of damped version of this type of absorber, known as a Stockbridge damper, which you often see on high voltage transmission lines near the suspension points. It reduces aeolian vibration.
  16. EnglishMuffin

    Wear and vibration in a bearing

    As electricpete says, you can sometimes detect one type of "wear", such as spalling/pitting caused by subsurface fatigue, by looking at the ball pass frequencies. This type of failure usually occurs suddenly, initially at one point on the raceway, and its onset is what one is calculating when...
  17. EnglishMuffin

    Natural frequency of a cantilevered beam with weight attached.

    Yes, if you're doing Rayleigh's method, that's generally the assumption you have to make, at least initially.
  18. EnglishMuffin

    Natural frequency of a cantilevered beam with weight attached.

    Actually, tomirvines equation is the same as one of the ones I quoted from Blevins, ie table 8-8 case 2. Adding on the extra piece of beam, (table 8-1 case 3) has the effect of reducing the natural frequency, and if the extra piece is very long compared to the first piece, there will be two...
  19. EnglishMuffin

    Natural frequency of a cantilevered beam with weight attached.

    Yes, that equation can be found in numerous places. But that wasn't the original question as I understand it. The mass is only part way along the beam !
  20. EnglishMuffin

    Natural frequency of a cantilevered beam with weight attached.

    I would go about it using the following approximate method. I would treat it as a two degree of freedom system, with two masses and two springs, as in Blevins table 6-2 case 3. I would solve for the lowest frequency - which means taking the negative sign in Blevins formula. I would choose the...

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