PSN-L Message - Subject: RE: How much mass ?

-----Original = Message-----
From: = psn-l-request@.............. [mailto:psn-l-request@............... On Behalf Of ChrisAtUpw@.......
Sent: = Thursday, February 14, = 2008 6:49 AM
To: = psn-l@..............
Subject: Re: How much = mass ?

In a message dated = 2008/02/14, shammon1@............. writes:
See comment = below:

= Hi, Jon, I like to use the old style Lehman design. The device I built with the = best results was the PSN San Jose Lehmans which used a 1-in square aluminum = rod L=3D 80cm and 80-ounce lead mass. They had a natural period of 20-seconds = with minimal setup effort. The total boom length was 100cm and they used a = brass plate mounted at the end of the boom for the damping. I was very happy = with the performance. One of the issues you will face is the selection of the = gauge of the upper support wire. I tried to use #8 machine (piano) wire but it = kept breaking during use. I increased the wire gauge to #10 machine wire and = the wire life was about 1-year before rust causes the wire to snap under = tension.

=

Hi Steve,

       If you use D'Addario piano wire, it = comes protected with Nickel plating.
       Use either Aluminum or Copper plate = for the damper. Al works, but 1/16" Cu is better.
       I use 1" square x 1/8" = thick NdFeB magnets for the sensor and 1" x 1/2" x 1/4" thick = NdFeB magnets for the damper, four off in both cases. A N/S pair on one = 1/4" backing plate faces a S/N pair on the other backing plate. The backing = plates are held in position by 1/4" zinc plated mild steel bolts. This = greatly reduces any stray field.
       Put the coil on the arm and the = magnet on the base. If you put a magnet on the arm, you will pick up a lot of = magnetic noise from the house wiring, passing trains and lorries etc.

Update = = 2/14/08: Thanks Chris, I’ll try mounting the = coil on the boom in the next revision. I never wanted to deal with the lead = wires off the rear of the boom. Do you have any tips for frictionless lead = wires?

Steve H. =

= When I moved here to Aptos, California I was forced to reduce the length of the = boom to L=3D60cm because of limited space as seen in the photos in the link = below and the resulting natural period is now typically 10-12 seconds. Needless to = say, the performance these Lehmans are marginal and I live near the ocean and = the wave action causes them to osculate during storms. =

=

       You should be = able to get 30 seconds out of a 60 cm arm quite OK. Most period limitations are = caused by a poor lower suspension, such as a point in a cup or knife edge. It = improves the period if you put the mass at the end of the arm.
       Use 3/4" square or round Al = tube for the arm - definitely not solid. You want to keep the arm light but rigid compared to the mass. I use 15 mm SS water pipe + plumbing fittings
       The bottom hinge should be ball on = a plane or crossed cylinder. You put the SS ball on the upright and the SS / WC = plane / SS blade the end of the arm. For WC rod, you can use 1/8" Tungsten = Carbide drill shanks, with the vertical on the upright. See www.smallparts.com. = You can also buy type 416 SS 1/4" shoulder bolts from McMaster Carr. www.mcmaster.com
       If you construct a double T frame = similar to http://www.bgs.ac.uk/education/school_seismology/seismometer.html you = can set up the system and then trim the response - all adjustments are = sequential, not inter related like on the psn designs. This makes setup and = adjustment far easier. The red block is a support, with a horizontal damping blade and = the sliding damping magnet is placed on the bottom frame. The coil is fixed = to the outside end and swings between the other magnet block. The diagonal Al = support tube prevents rotation.
       I used 3" x 1" Al U = Channel with triangular 6" x 6" x 1/8" Al plates to support the joint = in the prototype. This is easy to make and to set up. I use 1/4" SS = bolts.

My suggestion is to use = the calculation seen on this list to determine the boom length you desire. = Then plan the design based on that calculation. You said you wanted a device = with a period of 20-second and as pointed out below by Chris, L is based on the = length of the boom measured from the pivot point to the center of the mass = weight (Chris, do you have this calculation?). I have found that setting the = device up this way also reduces the impact of ground deformation (boom does not = remains level and centered) over time which improves the overall operation of = the device keeping the boom off the stops.
http://pw2.netcom.c= om/~shammon1/AptosStn.htm

       If the distance = between the hinge and the centre of mass is L metres and the suspension angle is = A, the period T =3D 2xPixSqrt(L / (9.81 x sin(A))) I use 0.56 m which would = give a 1.5 sec pendulum if held vertically. A is ~1/3 degree.
       Alnico magnets are now quite = expensive. NdFeB magnets are much cheaper and give a much higher output. For damper = and coil designs - click on Lehman at http://jclahr.com/science/psn/chapman/ = Make the damping blade wide so that it covers both magnets at all times as = shown.
       There is also an alternative design = under muventures.
       The boom does not have to be level, = but it should be adjusted parallel with the lower frame. Then you can simply = slide the damping magnet further over the horizontal damping blade to adjust the = damping.
       You adjust the whole rigid frame to = centre the arm and to set the period. I use three SS plates glued to the = concrete floor for the screw adjusters. I glued SS bearings into the end of the = adjust bolts. Then they don't move about / wander when you adjust them. The SS mounting nuts are glued to the underside of the frame.

       Regards,

       Chris = Chapman =

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