The water timer is an electronic device that allows or blocks the passage of water through it. Our purpose was to examine all parts of this object, analyze and determine how it worked and put it back together. After removing the covers and screws, it was determined that a rotating ball valve allowed or disallowed a clear passage for the water to travel through the timer. This valve was driven by an extensive gear train with a large gear reduction ratio, calculated to be approximately 2177. Each cycle of the water timer turned the ball valve one quarter of a revolution, either allowing or blocking passage through it. In this lab, a motor with a large speed and small force was tranformed by the gear train into a much slower speed and a greater force.
The water timer is a device often used by gardeners and grounds caretakers to ease the pains of lawn watering. It allows the passage of water through it or disallows it at appropriate times set by the operator. By inputting on the keypad a time to begin watering, the water timer will accordingly allow water to pass through it. This drives the gear train and turns the ball valve within the water timer, allowing a straight passage for the water to travel through the hose and accomplish its tasks. When this specified time is up, the gear train will turn once more, and once again the ball balve will rotate, ceasing the flow of water and ending the watering cycle. This cycle can be repeated at numerous times of the day or week. In more sophisticated versions of the device, a precise watering schedule may be devised and followed for the duration of an entire year or even longer. However, with the one discussed within, the timing mechanism is not nearly so advanced and does not allow an equal level of precision or such a complicated schedule. The purpose of this report is to clarify the internal mechanisms within the water timer and explain how and why they work the way they do to the end of better understanding its purpoes and function. In addition, to explain precisely how the device is operated. Each specific piece of the water timer is examined, analyzed, and discussed. As well, each part is neatly recreated with the assistance of computer drafting complete with dimensions [not here with Angelfire basic editor]. The entire mechanism has been recreated and redrawn, to the point of making it readable to a machinist for the purpose of recreation of the device. As a result, the water timer has been reduced to pieces and rebuilt, allowing it to be more fully comprehended and understood.
Water Timer (Part 1) - This is the entire unit, which consists of the Battery/Gear Box, the Battery Cap, and the Control/Computer Box. Its purpose is to water the lawn and is explained in much greater detail in the introduction.
Battery/Gear Box (Part 1a) - This houses the batteries and the gear train. It separates from the control/computer box and the battery cap. It is necessary because it holds the batteries and gears in place and allows viewing of the gear train from top and bottom when unscrewed from the control/computer box.
Battery Cap (Part 1b) - This is the lid for the battery/gear box. It comes off to expose two sides of the gear train for enhanced viewing. It also allows an easy away to change the batteries.
Control/Computer Box (Part 1c) - Houses the control panel on top, and the computer inside. This is where the user can press in commands such as “on/off”, and the computer inside will execute those commands.
Gear Train (Part 2) - Contains seven gears (going from motor to the last gear they are designated a, b, c, d, e, f, g respectively) Gears b, c, d, e, f, and g each have a large, 36-toothed portion on top and a small portion with 10 teeth welded to the bottom. Each small portion of a gear drives the large portion of the next gear (small b portion drives big c portion). This achieves a gear reduction ratio. This reduces the rotation speed and increases the torque from the motor to the last gear. It is necessary to turn the rotational speed of the motor into the torque needed to turn the ball valve connected to the last gear.
Ball Valve (Part 3) - Connected to last gear in the gear train, which spins it. It is a sphere with a hole through the center, with two holes on opposite sides. When the motor is run, it turns the holes to alternately appear and disappear from view. When the hole is visible, water it let through. It’s stopped when the hole disappears. Its purpose is to let water through or stop the flow, depending on the desires of the operator.
Motor (Part 4) - The mechanism is powered by the batteries and controlled by the control/computer box. It is connected to gear (a) and when the motor runs, gear (a) turns gear b. It is necessary because it provides the power to turn the gear train.
Gaskets (Part 5) - These are the rubber rings that insulate the inside of the water timer (1) wherever a crack occurs in the outer casing. Cracks occur where the Battery/Gear Box (Part 1a) meets the Battery Cap (Part 1b) and where the Battery/Gear Box (Part 1a) meets the Control/Computer Box (Part 1c). They are necessary to keep water out of the inner components of the water timer dry, like the computer.
1. Explain the basic operation of the unit. What happens when you push the on/off switch once? What happens when you push it again? How does the motor know when to stop?
The basic function of the water timer is to turn on sprinklers at a certain time. It does this by starting a motor (a), which turns six gears (a gear train) that achieve a gear reduction ratio, effectively turning the rotational speed of the motor’s gear (a) into torque in the final gear (g). The final gear (g) rotates more slowly and with more torque than the motor gear (a). Pushing the on/off switch once will make the gears move, and pushing it again will make the motor stop. The motor (4) knows when to stop thanks to a unique set up under the final gear (g). The final gear (g) has two protrusions, each one quarter the circumference of the gear. When rotated, the protrusions turn with the gear (g), and they depress or release a metal panel, which depresses or releases a knob, which makes the motor (4) stop.
2. What is the gear reduction ratio between the motor (4) and ball valve(3)? Explain your calculations. Why isn’t the motor (4) hooked directly to the valve (3)?
The gear reduction ratio equation is S2 = S1 x (T1/T2) where S1 = the speed of the first shaft in the train, S2 = the speed of the last shaft in the train, T1 = the product of the teeth in all the driving gears (drivers), T2 = the product of the teeth in all the driven gears (driven). When the last gear (g) goes one cycle, S2 is the number of motor gear (a) revolutions per one cycle of the last gear (g). 1 = S2 x (10 x 10 x 10 x 10 x 10 x 10)/(36 x 36 x 36 x 36 x 36 x 36), so S2 = 2177 revolutions/cycle
The motor (4) isn’t hooked to the valve (3) directly because the motor (4) wouldn’t have enough torque to turn the valve (3), and if it did it would turn the valve (3) much too fast.
3. How is water prevented from entering the inside of the unit?
There are rubber rings that insulate the inside from any cracks in the outer casing.
4. How many on/off cycles are required for the ball valve (3) to complete one revolution?
The ball valve (3) turns one quarter revolution every time you press the on/off switch, so two on/off cycles are required to turn it one revolution.
5. What needs to be considered in determining the motor (4) size? Why won’t a motor (4) with very little torque work?
You have to consider the amount of room left over after the gears (2) are in place versus the size of a motor (4) that will produce enough torque to spin the gear train (2). A motor (4) with little torque won’t be able to spin the first gear (a), since it’s bigger than the motor’s gear (a). If the first gear (a) doesn’t spin, none of them will.
6. What is the speed of the motor (a)?
We know the little gear on the motor (a) revolves 2177 times for every one revolution of the last gear on the train (g), so if we figure out how long one revolution of the last gear (g) takes, we’ll know how long one revolution of the motor gear (a) takes. By timing, our group determined that the last gear (g) spins a quarter-revolution every four seconds, or one revolution every 16 seconds. The following equation will find the rpm of the motor gear (a)
(1 revb / 16 sec)(60 sec / min)(2177 reva / revb) = 8163.75 rapm where reva = the motor’s revolution and revb = the last gear’s revolution Using the equation “(1 revb / 16 sec)(60 sec / min) = 3.75”, we find the last gear goes at 3.75 rpm.
7. Assume a motor is powered as 1 Nm, spinning a gear at high speed but low torque. The driving gear is meshed into a series of 6 gears, each smaller than the previous. The final gear is connected to a weight of unknown mass. Calculate and determine the weight of the unknown mass. Use the gear ratio obtained from your dissection to provide a realistic simulation of your water timer.
The first gear (1) has an rpm of 8164 and revolves with a force of 1 Nm and it has a radius of .05 meters. You can find torque using the equation T1 = Fr1 where T1 is the torque, F is the transmitted force, and r1 is the radius of the drive gear (in this case the only gear). Using this equation, gear 1 has a torque of .05 Nm2.
Using the equation T2 = T1 x (r1/r2), where T1 is the torque of the driving gear, T2 is the torque of the driven gear, r1 is the radius of the driving gear, and r2 is the radius of the driven gear, we can find the amount of torque transferred between two gears.
Torque measured in Nm2 The final gear (7) revolves with a force of .05 Nm2. Using the first equation (T1 = Fr1) we can find the force it can lift.
.05 Nm2 = F x (.05m), F = 1 Nm
One Nm equals 9.8 kg, so the gear train can lift a mass of 9.8 kg or less.
My team was an excellent team. I had no difficulties with them because we all had a focus on making a good project together.
David Oliver - A good worker with good communiction skills. He had no problem telling us his opinion and ideas. He was always willing to help in any part of the project. The only thing I can suggest is that he tries to be on time to the team meetings. His tardiness was not a big problem becuase he was never more than ten nimutes late and once he came in he did not slow the group down and immediately jumped into the subject at hand. David was great to work with and he also showed us many things among them he showed us what it was to write well.
Matt Meehan - He is a very hard worker and deserves much credit. Matt was very anxious in helping with the project. This was noticeable because Matt was always the first to offer in doing any job that came up. Matt was always willing to do the work in fact I believe he would have done the whole project if I had let him. He is an over achiever and was on time to every meeting in fact many times he was early. Matt had many very good ideas and thoughts during the course of the project, but the only problem was that I had to ask him most of the time for his opinion. The only suggestion I have is that Matt should speak up more often because he has many good ideas worth listening to. Matt was a great contribution to this team many times helping us with has very special abilites for example he quickly figured out problem 7 which was giving David and I a huge headache.
David did the abstract and intro, I did the description and discussion (which is missing a picture and two little charts), and Steve was team leader and did the drawings, which aren't shown here. We got a "meets expectations" on this project, which is like a high B or a low A.
