I should elaborate a bit on the counterbalance spring forces, since most hatch counterbalance springs are torsion springs implemented as an intended moment acting on the hatch hinge.
This moment
M [N * m] = k [N * m] * theta [radians]
is typically applied via short spring ends acting at an effective distance from the actual axis, so you can work back to the distance you are interested in (presumably at the hatch center of mass),
since F_1 * r_1 = M = F_2 * r_2. Ideally, you want the required counterbalance force to develop over a spring twist of 90 degrees (pi/2 radians) so that you have full counterbalance (for safety reasons, slightly less than the actual force required to open the hatch) with the hatch closed, and zero counterbalance with the hatch open. Note that the relationship is not precisely linear, as the counterbalance force in this case varies linearly with hatch open angle, but the force acting to close the hatch (due to gravity) varies with the cosine of the open angle.
-Sean
On Nov 4, 2009, Sean T. Stevenson <cast55@telus.net> wrote:
1) For force due to material weight:
F [N] = m [kg] * a [m/s^2]
Where m is the mass and a in this case is acceleration due to gravity: 9.80665 [m/s^2 ].
2) For force due to pressure:
F [N] = P [Pa] * A [m^2]
where A is the projected area of the hatch in the plane under consideration. If you have a dome hatch, you only need consider the forces in the direction of the axis, since the radial forces cancel each other.
3) For force due to counterbalance spring:
F [N] = k [N/m] * x [m], where k is the linear spring constant and x is the compression / extension distance, OR
F [N] = k [N] x theta [unitless], where k is the torsion spring constant and theta is the twist angle, expressed in radians.
4) For force due to buoyancy, equal to the force due to weight of the displaced water (using equation 1 above):
To figure the mass, m [kg] = rho [kg/m^3] * V [m^3]. Use densities of rho=1000 kg/m^3 for fresh water, or rho=1025 kg/m^3 for salt.
-Sean
On Nov 4, 2009, Alan James <alanjames@xtra.co.nz> wrote:
************************************************************************ ************************************************************************ ************************************************************************ The personal submersibles mailing list complies with the US Federal CAN-SPAM Act of 2003. Your email address appears in our database because either you, or someone you know, requested you receive messages from our organization. If you want to be removed from this mailing list simply click on the link below or send a blank email message to: removeme-personal_submersibles@psubs.org Removal of your email address from this mailing list occurs by an automated process and should be complete within five minutes of our server receiving your request. PSUBS.ORG PO Box 53 Weare, NH 03281 603-529-1100 ************************************************************************ ************************************************************************ ************************************************************************Hi psubbers,I'm trying to calculate what the lifting force will be on thedome in my hatch when it is just submerged; but aren'tquite sure what I have to take into consideration to calculate this. Any thoughts on this?Thanks, Alan