Seismometer Pivot Studies  2008/9/7


   Charles R. Patton

Chris Chapman

Brett Nordgren



 The amateur seismology community has experimented with many variations of horizontal motion sensitive seismometers.  The essence of most of these variations can be reduced to combining a mass with a pivot system that allows the mass to describe the trajectory of a pendulum of very long period, preferably in the 10 to 20 second region.  Experience shows that a critical part of this construction is the pivot system used.  Friction is particularly onerous for a high sensitivity, long period pendulum.  Many amateurs have had stability and adjustment problems, with the causes not always clearly defined.  It was decided to explore the geometry of some pivot systems to see if there might be geometry errors inherent in some of the pivot systems as they were adjusted to long period.  This study was undertaken to develop exact equations for some of the common systems allowing spreadsheet comparisons to provide insights into the pivot geometry and to explore some possible alternatives for future experimentation.  

The Pivot Systems Studied:

1) Ball on Plate

2) Plate on Ball

3) X-Roll

4) X-Suspension

                 with additional comments on:

5) Wire flexures

6) Bendix flexures

 7) Rollamite bearings



 Ball on Plate:               

 Most constructions seem to end up with this class of pivots in a Lehman gate style seismometer.  Instructions may go like, “Put a hardened piece of steel such as a razor blade or tool steel on the upright, and a hard point such as a ball-point pen tip (with tungsten ball) in the end of the beam” for a low friction point.  The biggest drawback to the ball on plate pivot is the terrible tendency for the pivot to slide sideways on the plate due to the side force of the force vectors when the beam swings out of perpendicularity with the plate with as little as 4.5 degrees deflection.  Tremors certainly don’t cause swings of this magnitude, but many long period Lehman have significant tilts due to temperature, base shift, etc. and can easily exceed these values.  In order to control this, most instructions suggest that the plate be “dimpled” with a punch or some such.  But a dimple defeats the concept of low friction as the geometry invites the possibility of the ball having to “slip” in the dimple in order for the beam to swing.  This is probably one of the worst choices, yet is the one of the most common type of pivots used.  A variety of materials can be used.  SS, tungsten carbide and sapphire spheres are available. The cheapest material for a counterface is to use a SS scalpel blade. Triangular tungsten carbide tips for lathe tools are available, but buy the un-coated sort.

Some other factors which may also give problems in amateur seismometers:

1)              A failure to design the suspension in such a way that there are no vertical forces/loads trying to move the ball away from it's point of contact. Any vertical load is liable to cause position creep in operation, if not a catastrophic failure when a severe quake is experienced.

2)              A failure to provide a sufficiently hard counterface. High tensile bolts are not hard enough. The counterface needs to be fully hardened martensitic SS, tungsten carbide, or sapphire.

3)              A critical factor is trying to use bearings of too small a diameter. For axial loads of 1 to 4 pounds, the minimum diameter is ~1/4".  3/8" to 1/2" OD bearings are preferred. Note that the axial load on the arm is likely to be ~double the mass weight.

4)              Due to the ball rolling sideways in normal operation, the center of rotation is not quite constant with changes of deflection, although the errors are not large enogh to be a practical problem.  The spreadsheet allows insight into this phenomenon. 


See ball on plate study.xls or ball on plate study.pdf for calculations.



Plate on Ball                                                                    

       The simple reversal of position of the ball and plate has significant positive effects.  The locus trajectory error is more than 4 times better than that of the ball on plate for comparable sizes.  The tendency to slip is almost entirely eliminated due to much better geometric properties.  In a test, the arm was stable at angular deflections of 45 degrees.

For systems which use a plane rolling on a ball, the angle of rolling contact that can lead to pivot point skidding is reduced from the arm deflection angle by a factor of R / (R+L), where R is the radius of the ball and L is the radius of gyration of the mass.  For a 3/8" ball and a 22" arm, this is this is 1/60.

 See plate on ball study.xls or plate on ball study.pdf for calculations.



Crossed Cylinder Suspension.

 The geometry can be viewed as a variation of the “plate on ball” if configured as described here.  These suspensions can provide a greater load capacity than a sphere of a similar radius of curvature.  The rolling contact loss is lower than that of a sphere on a plane.  These can also make an excellent suspension for a long period Lehman. The axis of rotation is well defined when the vertical rod is mounted on the support column and the cross rod is mounted on the end of the arm.  Common materials are polished hardened martensitic SS and polished tungsten carbide rods.  It is possible to buy surplus tungsten carbide drills for circuit boards and use their shanks.  Tungsten carbide center drills and needle rollers are also available.




 During the research for this note, the X-Suspension was found as a solution being used in parts of the LIGO (Laser Interferometer Gravity-Wave Observatory) equipment for vibration isolation of the detectors.  So this was added as a possible future avenue for a seismograph suspension.  Like the folded pendulum, a horizontal trajectory can be easily adjusted.  The downside is that due to the hanging nature of the suspension, it has some sensitivity to the orthogonal axis.  This may be acceptable if one uses a sensing system that is insensitive to the orthogonal movements such as Dr. Randall Peters' linear capacitance sensor.  Also it may be possible to implement it with an interesting implementation of the X-roll pivots (see X-roll next).  Be aware that the equations solve for a flat trajectory of the center of mass, but as that center of mass moves, there is a significant twist of the mass during the swing.   

See Barton X-pendulum study.xls or Barton X-pendulum study.pdf




 During a discussion on the PSN discussion list (webtronics.com ), a series of experiments were executed to try to find low friction pivots.  These experiments included carbide rods rolling on other carbide rods (crossed cylinder pivots) as discussed above.  But the pivot that came out the best in that series of experiments was the X-roll.  The study of its geometry also shows it to be one of the best with low geometry error.

 In contemplating various seismometers using X-rolls as pivots it is likely that that the X-suspension (see above) could be implemented as half X-roll pivots with only 3 band/flexures total.  A folded pendulum using X-rolls would need 4 X-rolls with 12 bands total.  A normal Lehman could be implemented with two X-rolls and 6 bands.  I.e., generally the X-roll can be used to replace pivots and flexures in other types of seismometers as long as the rolls can be kept in tension.

See X-Roll study 2.xls  or   X-Roll study 2.pdf



Wire flexures

Very often wire flexures are used because of their simplicity, but there are some major disadvantages: 

1)               The end surface of the clamp plates need to be machined and kept in alignment to a small fraction of the diameter of the wire if lateral offsets/asymmetric bending effects are to be avoided. They need to be made of material at least as hard as the wire. Collet mountings are preferred as opposed to pairs of flat plates.

2)               Due to the high local stress and the short bending radius, the frictional/anelastic losses are higher than in the bearings based on rolling contacts.

3)               Due to the high stress values wire fracture failures are to be expected.  The wire is typically very highly stressed close to the jaws of the clamp. It has to resist the clamp forces providing the axial load as well as flexing.  This can lead to fatigue failures. If the length of a used suspension wire needs to be adjusted, the whole wire should be replaced

4)               The high stress of clamps can also cause bending and bending causes hysteresis.

5)               The geometry of the trajectory is a complicated stress/strain relationship, often determined by trial and error. 

6)               The wire can have severe secondary resonances and little resistance to orthogonal forces.

7)               Due to the requirement to minimize the wire diameter, it may be relatively elastic to changes in axial loads, so the overall length should be minimized.  It can also respond to resonance effects like the string of a musical instrument.

 Note that D'Addario can supply nickel plated 'piano' steel wire. This can be obtained from 0.008" diameter upwards and it is resistant to corrosion.



Bendix Pivots

 Also known as flexure pivots or cross-flexures are now available from http://www.c-flex.com/.  They are subject to significant, but well defined, center shift. See the URL for C-Flex for more information.  This center shift can be mitigated by using very small gaps between the clamps such as suggested by S-T Morrissey in his designs (see crossed foil later in this paragraph), but this leads to higher stress/strain friction and possible fractures.  The positive quality of these flexures is their good control and repeatability of position.  They can be extremely stiff in the orthogonal axis.  Repeated flexing which only slightly exceeds the elastic limit (as with crossed foils) will result in hysteresis effects, possibly creating the results which Randall Peters is investigating, and certainly adding a damping effect.  The commercial flexture pivot design specifies a significant hysteresis.  The variation of a crossed foil suspension is quite rigid when in tension, but it cannot resist compression with thin foils. With wide foils, it can have an appreciable resistance to axial loads.  However, the foils tend to bend sharply near the clamp edges. This causes it to have some internal friction loss and it suffers from anelastic noise. The centre of rotation is well defined but travels around a locus that is not the cross-over point of the foils.



Rollamite Bearings

 Rollamite Bearings are also a low loss special case of the X-roll bearings where the gap is zero.  Generally they’re used in reverse where the lower roll supports the weight of upper roll and its attached mass.  There are variations where the bands can be configured in tension to keep the rolls in contact while hanging the load.  They are generally rigid in the orthogonal axis, but their biggest concern in seismology use is that they might trap particles between the rolls leading to stiction/hysteresis effects.



Some Final Comments


Geometrically, all pivots seem to have available a locus of sufficiently low theoretical error in their normal implementation modes that this is unlikely to be the reason to choose one over the other.  But there are other points that do make some choices much better.  In particular the stability and repeatability of the geometric pivot point along with its rigidity in the orthogonal directions are important along with low friction/stiction loss.

In the attached papers of some actual pivot tests, loss of crossed rods and loss of zero torque suspensions, Meredith Lamb found that rolling foil suspensions came out tops in the low loss tests followed by crossed wire/flexures that are ~ 2x better than balls on flats (or vice versa) that are ~ 2x better than crossed rods.

Other experimenters have found rolling foils are  better than rolling wires, which were better than crossed cylinders, which were better than ball on a plane, which were better than ordinary crossed foils, which were better than single foil Cardan suspensions.

Unsatisfactory 'suspensions'.

Do NOT use either a 'point in a cup' or a genuine metal knife edge on a flat for a suspension system. Both apply stresses over or near the maximum strength of the metal and even if they work initially, they will not last long.  If the edges do not either roll over or shatter, they will dig into the counterface.  These systems are a common cause of poor performance in amateur seismometers.

The 'knife edge' suspensions used in chemical balances are 60 degree triangular agate rods, but the edges are lapped to give the action of a very small cylinder rolling on a plane.


 Notes On Using The Spreadsheets

The spreadsheets available here make possible the quantification of the locus errors of many of the popular pivot systems.  Note that they require of bit of guess work.  The equations figure the exact path of the bob and return a comparison to the path generated by the locus point you enter.  You have to tweak(optimize) the locus point to minimize the error being indicated.  Once you’ve done a few estimates, it should only takes a few minutes to home in to the needed locus point.    The spreadsheets are not locked so you can experiment with them as you please.


Enjoy.  Experiment!  


last edit 2008/10/15 ref corr X-roll docs