MODEL BOAT UNIVERSAL JOINTS
I can hear the voices emanating form the work sheds and shops again …….. ”universals! Which, what , how, why?”
This is a subject that many tend to gloss over and a few ponder. It’s a topic I don’t think I’ve seen/heard anyone delve into, why do we use them, what is the theory behind them, what s the best to use for my application. With this tutorial I’m going to attempt to clear up some of the above questions about these mysterious little necessaries in the hulls of our vessels.
DEFINITION
What is a universal joint by definition?
A universal joint, universal coupling, U joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a joint or coupling in a rigid rod that allows the rod to 'bend' in any direction, and is commonly used in shafts that transmit rotary motion. It consists of a pair of hinges located close together, oriented at 90° to each other, connected by a cross shaft.
HISTORY
The Universal joint is recorded to have been invented by an Italian mathematician by the name of Gerolamo Cardano in 1545.
In 1678 and English man wrote of universal joint for Cardano’s device in his book called ‘Helioscopes’. Since then the names Cardan(o) and Hooke have been linked to the universal joint device. The Hooke or Cardan joint was initially used as a single element in a shaft with a an angular displacement and resulting non uniform rotating speed across the joint. Hooke proposed using a second Hooke joint on the same shaft, shifted by 90 degrees, to eliminate this variation in rotating speed. Although it wasn’t 100% successful, it did reduce the rotational speed problem. A later CV or constant velocity joint eliminated the speed difference entirely.
In model boating, most off us have used a single universal (Hooke, Cardan) joint to connect our motors and prop shafts. In many cases this is attended by excessive noise and vibration with queries as to why? With a single Cardan joint, the two shaft ends have to be perfectly aligned. Any discrepancy in alignment between the ends of the shaft will cause binding with an accompanying vibration and noise. A single Cardan joint can allow one shaft to have a slight (usually no more than 15 degrees) angular misalignment. However, the two ends of the shaft must be perfectly aligned as the center block on the single Cardan joint won’t allow for horizontal or vertical displacement. Only a double Cardan joint can allow for vertical or horizontal displacement to some extent.
Shaft joints could be can be offset in four different ways. (see below):
Any one of the above discrepancies in alignment will cause noise and vibration when our model operates. In order to get a truly aligned shaft, these discrepancies have to be taken out of the alignment of the two shafts. Putting in a coupling shaft with two Cardan joints will compensate for some of this, however, the closer two shafts can be aligned, the quieter, smoother running the motor/shaft combination will be and the less amps the motor will draw.
Automobiles, at least the older ones with rear wheel drive, use a drive shaft with two Cardan joints. Like our boats, they wouldn’t work very well with on Cardan joint. Front wheel drive cars use constant velocity universals, a different animal from the Cardan joint.
SHAFT ALIGNMENT
On larger shafts dial test indicators, verniers and feeler gauges are used to set alignments of shafts accurately. That isn’t really possible on our small cramped quarters, so other options have to be looked at.
The simplest, but not necessarily the most accurate method of shaft alignment is visual alignment of the two shafts, taking into consideration the four directions a shaft can be out of alignment. This method will certainly get you close enough into the ball park to allow the use of a double Cardan shaft to take out most of the differences between the prop shaft and motor.
A more accurate method would be to slide a piece of firm tubing, such as brass, over the motor shaft and prop shaft. This will hold the motor and prop in alignment until you have the motor mount permanently fastened in its proper location. Remove the motor from the mount and install the double Cardan shaft. (note: we will look at motor mounts in a future section of this tutorial).
Another method of checking alignment of motor and prop shaft is to install the motor and connect it to the prop shaft as is normal. Allow the motor mount to be adjustable as to position, or allow the motor to be adjustable inside the mount. Set up an electric supply to the boat motor with an ammeter in series with the motor leads. Bring the motor up to a moderate speed and watch the amp load on the meter. Adjust the motor position to get the lowest amp reading (you’ll note quite a change in motor noise as you do this; it gets quieter as you get closer to the correct alignment). When satisfied that you have the lowest amp reading and noise level, fasten the motor into position and recheck. This method can provide a very accurate alignment for our small vessels with a minimum of hassle.
MODEL UNIVERSAL JOINTS
What’s available for model boats? Most use the single Cardan Huco joint for model boat applications. However, as you’ve seen above this is not always adequate. Two Huco joints back to back or on the ends of a shaft 90 degrees offset will work. Huco also make a conversion to take two of their Cardan joints with a center piece to make a proper double Cardan shaft. These can work very well. Graupner offer a similar item but in a slightly small size that is adequate for most vessels.
Dumas make a nice simple little unit consisting of two brass ends with a preformed nylon wishbone. This gives the double Cardan effect with a 90 degree offset. They’re reasonably priced and simple to install. The design of the ends allows the wishbone to move and take up any differences in distance between the motor and prop shaft.
As has been pictured above, there is also the rubber coupling. These can work well in vessels with prop sizes less than 1 ½” diameter. They are quiet, when properly set up, and easy to install.
Graupner double Cardan joint
Huco single Cardan joint
Dumas double Cardan joint
scratch built double Cardan style with dog bone
I hope this gives some insight into model boat universal joints and the correct way to go to get them to work properly. I all answer any questions I can.
John
Author
Topic: Universal & Flexible couplings (Read 46780 times)