Introduction

A universal joint is a type of mechanical instrument used in many applications to transmit rotation through slightly misaligned shafts (See Figure 1). The misalignment correction is limited by the design of the shaft, but can be amplified by use of multiple universal joints.(1) Correction is needed because if the junction between shafts is not straight, rotation becomes jerky and unreliable due to inconstant angular velocity(2). However, there still exists an inherent problem in universal joints because not all components have the same angular velocities over one rotation(3).

Figure 1 (from reference 4)

A typical universal joint has a four point cross which is connected to two shaft yokes at right angles to each other (see Figure 2).

Figure 2 (from reference 3)

History

The ancient Greeks invented the concept around 300 BC, and 16th century Italian physicist Geronimo Cardano invented the modern form to hold a compass horizontal in ships. The name "Cardan joint" stuck, and the joint was so called when Robert Hooke (of Hooke's law fame) improved the fundamental design in the 17th century, although Hooke tried to change the name to "Hooke's joint". It was not called a "universal joint" until Henry Ford started using the pieces in automobiles, and in some modern applications, especially in Europe, the pieces are still called Cardan joints. (3,5)

Figure 3 (from reference 3)

Today, universal joints are used universally in many machines in which there are long rotating shafts. Most notably, such joints are found between the transmission and rear drive shafts of rear-wheel drive cars (see Figure 3) and in old-fashioned tower clocks.

Universal Joints in our Clock

In our model, the universal joint is 3.5 inches across the four point cross' arms (see Figures 4 and 5). One universal joint sits at the top of the clock for each face to align the shafts that control the clock hands. At this point in time, we are uncertain as to whether or not there will be another universal joint by the minute and hour hand drives at the clock faces. We hypothesize that there might be another pair to further help correct alignment, but we can not be sure because we are missing some parts, most notably the long shafts that transmit rotation from the clock frame to the clock faces.

Figure 4	Figure 5

(1) http://autorepair.about.com/od/generalinfo/l/bldef806.htm
(2) http://www.nuigalway.ie/mechbio/downloads/FinThMachines3.doc
(3) http://www.4crawler.com/4x4/CheapTricks/Driveline-101.shtml
(4)http://en.wikipedia.org/wiki/Image:Universal_joint.gif (5) http://www.mustangmonthly.com/howto/mump_0712_how_to_choose_universal_joints_and_driveshafts/index.html