Skip to content

3DLC0230

Jump to bottom
Karl Lew edited this page Jan 10, 2015 · 11 revisions

Catenary Spring

Function

The catenary spring functions as an extension spring with a range of travel slightly less than the height of the arch. Although helical springs are common, their sheer variety can make for a daunting shopping experience. Since 3D plastic tends to be springy, the catenary spring offers a simple, easily replaced alternative to the common helical spring. The form of the arch relies on the catenary curve, which distributes sheer stresses evenly throughout the arch--other curved shapes such as ellipses tend to focus stress unevenly. Notably, the arch of the spring can be used to good effect in designing for part clearance.

A pair of catenary springs holds a pair of effector rods on their ball studs. The catenary springs also keep the rod ball stud sockets facing each other. One catenary spring holds top of the rod pair on the pulley arm ball studs. The other catenary spring holds the bottom of the rod pair on the effector ball studs. The arch of the top catenary spring clears the limit switch interruptor, which may collide with a straight spring. The arches of all three top catenary spring must clear the bottom of the corner brackets of the top plate. The springs must be stiff enough to keep the effector rods on the ball studs but not be so tight that the ball studs bind up. In addition, the catenary springs permit rapid (dis-)assembly of the effector rods with a single hand.

Each catenary spring is composed of two catenary arches which separate under extension load (see picture). Earlier versions of the catenary spring using a single arch were too loose and fragile--simply thickening the arch proved ineffective. Using two arches stiffens the spring and adds strength. The spring end pivots are slightly offset from the ends of the arch so that they act as levers on those arms for mechanical advantage.

Manufacture

The novel use of 3D printing to create springs introduces some challenges:

  • The catenary spring has been print tested with PLA using a 0.35mm nozzle
  • For best filament bonding, print at higher temperature if possible
  • For best filament bonding, print at slower speeds (e.g., 50%)
  • For reliable pinholes, print at slow small perimeter speeds (e.g., 25mm/s)

Materials chosen will vary in stiffness (e.g., orange vs. black PLA). For this reason, various spring lengths are provided. Choose the longest spring length that is:

  1. Stiff enough to hold the effector without wobbling
  2. Loose enough to handle effector rod (dis-)assembly without breaking

Assembly

See picture for assembled spring:

  1. Check that the spring has enough room to pivot freely in the effector rod spring holders, sanding away excess plastic from the effector rod spring holders if necessary
  2. Insert a length of 1mm wire (e.g., paper clip) as a pin through the effector rod spring holders and one of the catenary spring end pivots
  3. Repeat with other end of spring and second effector rod
  4. Crimp the wire ends down (towards the effector). Do not crimp them up--they will collide with the spring.

See Also

Clone this wiki locally