# Lifting Bridge part 3

## Lifting Bridge. 3.

3.4.5 Calculating the total transmission ratio

• The multiplier transmission in figure 23 is composed of two transmission reactions, each one with a transmission ratio of 3:1. If we combine both the ratio is multiplied and the final ratio is 9:1. In such as case we can talk about a total transmission ratio (iT) since two or more single transmissions are represented in the calculation. The formula used to calculate the total transmission ratio are: iT=  Na / Ne “Na” represents the number of turns of the entry wheel, wheel a, and Ne, is the number of turns of the exit wheel, wheel e.
• iT=( Z2 x Z4 x Z6 x ….) : (Z1 x Z3 x Z5 x ….. )
• Where z represents the number of teeth on each wheel. iT= i1 x i2 x i3 xi4… Where i represents each of the single transmission ratios. In the case of figure 25 where Z1 and Z1′ 0 13 teeth and Z2 and Z2′ =39 teeth i = Z2/Z1 = 39/13 = 3;  i’ = 39/13 = 3   iT = 3 x 3 = 9

Results

Our mechanism in figure 24 is non-multiplying which means that the turns of the last wheel are reduced while its power is increased.The transmission ratio is as follows iT= (Z2 x Z4 x Z6 x Z8 ) : ( Z1 x Z3 x Z5 x Z7 )

= (50 x 50 x 50 x 50) / (10 x 10 x 10 x 10 ) = 625:1

3.5 Assembly of the different parts

3.5.1 Fix the motor to the base (1) with 4 2.9 x 9.5 mm screws (36), as shown in figures 29 and 30.3.5.2 Attach the connection  strip with two 2 x 21mm screws (20) and fix the battery holder (39) in place with two 2.9 x 9.5 mm screws (36). See figures 29 and 30.3.5.3 Attach three 40 x 15 x 10mm pieces of wood, obtained in point 3.2.9, together to form the counterweight. Screw two  eyebolt (35) into the counterweight as shown in the figure. Then loosen open the side bolt with a screwdriver in order to be able to pass the string through it later. 3.5.4 Screw the other two eyebolt  into the back part of the left support. See figure 25. Then tie the end of the cord to each one, so that the cord is tense. The cord will act as a guide for the counterweight.3.5.5 Cut a piece of string of about 40cm and tie a knot at one end. Pass the string along the groove in the crosspiece in the centre of the bridge and pull until the knot makes contact with the groove. Tie the other end of the string to the eyebolt on top of the counter weight. Place this near the top of the vertical piece of wood and pass the cord through the cleft of the pulley.Place the guiding string in the open eyebolt on the side of the counterweight and test it by raising the bridge manually. When the bridge is up, the counterweight shouldn’t be touching the base.3.5.6 Tie a knot at one end of a 70 cm long piece of string and pass it through the other groove on the crosspiece in the centre of the bridge. Stretch out the string until you find the end and tie the far end to the shaft to which the red wheel of the motor is attached. This shaft will function as a winch, winding the string round itself.File the mounting shaft down a bit and attach the knot in the string to the shaft with hot fusing glue. This will ensure that the knot doesn’t slide on the shaft.Pass the string through the pulley winch and make the motor turn (with a battery) so that the string wraps round the shaft until it is tense but not yet lifting the bridge.

3.6 Electrical installation.

The connection of the wires (27) to the motor, to the sliding switch (25) and to the limit switches  (24) will be done using soldering, at the appropriate terminals. It’s convenient to solder the end of the wires that connect to the connection strip to assure a good level of contact. In the electrical plan, figure 26 shows the connections that must be made. Before starting the installation of the wiring, assign a colour to each wire (C1,C2, C3…). This way, it will be easier to make the connections and controls. The wires and their positioning are shown in figures 27 and 28.When the installation of the wiring is finished and its correct functioning has been checked, you can attach the wires to the appropriate pieces of wood with warm fusing flue at small intervals.

4 How it works

Place the batteries (not provided) into the battery holder to which the (40) will be connected. When the bridge is in the horizontal position, the green LEDs on each side of the bridge ought to be lit up and the motor should be stopped.When you move the sliding switch the motor should start to work and the string should roll itself up round the winch.When the bridge starts to lift the barrier should go down and the green LEDs should go out while the red LEDs simultaneously light up.When the maximum elevation is reached, the motor must be stopped by the limit switch . The Leds shouldn’t change.Upon moving the sliding switch again the motor should start again in the opposite direction and teh bridge should lower. The LEDs will stay the same.When the bridge reaches the horizontal position the barrier should lift, the red LEDs should go out and the greens, simultaneously, light up, all of which is caused by the inferior limit switch.If the bridge doesn’t work as described above, there must be a connecting error and you should recheck the connections and wiring, following the diagram and making any necessary changes.The direction of the motor’s turns may invert itself thereby changing the connection of the terminals in the motor or of the sliding switch.If the LEDs don’t work, check to see if you have set them up with the correct polarity.If the motor stops too soon or too late, move the limit switches backwards or forwards.If the motor jumps or stops, check the mechanism.If any piece of wood is rubbing against another, rub it gently with sandpaper.

Dictionary::::

brass :  any of various metal alloys consisting mainly of copper and zinc