## PSN-L Email List Message

Subject: Re: Crossed rod hinge/suspensions v/s ball bearings hinge/suspensions
From: ChrisAtUpw@.......
Date: Wed, 26 Oct 2005 20:29:01 EDT

```In a message dated 26/10/05, KROBERTS23@............... writes:

> Hi, I am the one who proposed the double shim hinge for a horizontal
> seismometer. However the string method would seem to have less torsional resistance
> than the double vertical shim. Will the string / filament support the
> horizontal pendulum 'boom'? (or will boom weight allow the vertical vector) gravity
> allow the boom to drop far enough to significantly the angle the once
> horizontal string / filament enough to increase the angle of the mast end of the
> pendulum and affect it's torque? I don't know if increasing the angle will
> adversely effect / increase the torque needed to allow easy horizontal motion to
> the pendulum or if the arrangement will decrease the needed torque forces to
> swing the pendulum. I feel the wire / string / filament will increase the
> needed torque to move the pendulum,

Hi Geoffrey,

If you make the suspension with a wire around circular rods, the
bending forces exactly balance and stay constant as the suspension rotates. This is
not true of 90 deg crossed foil / wire hinges which have the ends of the foil
/ wire clamped.
The 'traditional' crossed foil hinges will take an appreciable end
load, but as you mention, crossed wires may sag a bit, depending on the vertical
load on the hinge end of the boom. The easy way around this problem is to
connect the top hinge onto the end of the boom with a light tube to maintain the
separation and the alignment of both, but have the top wire loop set at an
angle to the horizontal to take the vertical load.

Regards,

Chris Chapman

In a message=20=
dated 26/10/05, KROBERTS23@............... writes:

Hi, I am the one who propos=
ed the double shim hinge for a horizontal seismometer. However the string me=
thod would seem to have less torsional resistance than the double vertical s=
him. Will the string / filament support the horizontal pendulum 'boom'? (or=20=
will boom weight allow the vertical vector) gravity allow the boom to drop f=
ar enough to significantly the angle the once horizontal string / filament e=
nough to increase the angle of the mast end of the pendulum and affect it's=20=
torque? I don't know if increasing the angle will adversely effect / increas=
e the torque needed to allow easy horizontal motion to the pendulum or if th=
e arrangement will decrease the needed torque forces to swing the pendulum.=20=
I feel the wire / string / filament will increase the needed torque to move=20=
the pendulum,

Hi Geoffrey,

If you make the suspension with a w=
ire around circular rods, the bending forces exactly balance and stay consta=
nt as the suspension rotates. This is not true of 90 deg crossed foil / wire=
hinges which have the ends of the foil / wire clamped.=20
es will take an appreciable end load, but as you mention, crossed wires may=20=
sag a bit, depending on the vertical load on the hinge end of the boom. The=20=
easy way around this problem is to connect the top hinge onto the end of the=
boom with a light tube to maintain the separation and the alignment of both=
, but have the top wire loop set at an angle to the horizontal to take the v=

Regards,

Chris Chapman

```