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Basic Mechanical Output

DC Motor Types
Continous vs. Stepping
When shopping for motors you will find motors listed as both continous and stepping. The names imply the operation fo these motors fairly literally. Continous motors provide continuous rotation of the motor shaft when given power, while stepper motors only turn a few degrees every time power is applied to the motor. Stepper motors require additional circuit components to be built but offer a higher degree of accuracy for many applications. For now we will stick with controlling continuous motors.

Servos
A special subset of continuous motors that allow a feedback loop between the motor and the microcontroller that provides precise motor control.

Motor Specs
Operating Voltage
All motors are rated by operating voltage. In many motors this is a range between two voltages. ex 1.5 - 6 V. Most motors are robust enough to be operated fairly outside these specs. This will effect motor operation such as torque.

Do not run a motor 50 percent under recommended voltage.
Will fail to run.
Do not run a motor 30 percent continuously over recommended voltage.
Will overheat.

Our Motors:
Voltage = 7.2vdc

Current Draw
Current Draw is the amount of current in milliamps and amps that the motor requires from the power supply. Current draw becomes much more important when describing load or work. The current draw of a free spinning motor may be quite low. The same motor doing work can jump as much as a 1000%.

The motor will demand more current from the power supply as load is added. Some motors are marked with this rating which is refered to as the "stall" rating. It is a built in failsafe that does not comprimise the motor by having it shutdown before shorting out. When metering motor current make sure the setting is in the 5 to 10 amps region on the meter.

Our Motors:
Current (at 7.2v no load) = 200mA
Current (at 7.2v locked shaft) = 3.80A

Speed
The rotational speed of a motor is given in rpm. (revolutions per minute) Most DC motors haves speeds outside our usefulness. We hav aquired geared motors that bring these speeds down to 150 rpm.

Our Motors:
RPM = 175

Torque
Torque is the force exerted upon its load. The higher the torque the larger the possible load at a faster operating speed. By reducing the torque and the motor will slow under the workload. Add too much load and the motor will stall.

Our Motors Ara Rated:
Stall Torque = 99.04 oz-in (7.1 kg-cm)

There is not a standard torque measurement. There are ways to measure torque using a lever attached to the motor shaft with increasing weight on the load end. For the puposes of these exercises it is best to to simplier load tests by securing the motor in a table vice and providing variable resistance using a set of clamps.

Transfer of Motion
Most of you will use motors as your actuating element. There are many ways to transfer this motion mechanically to produce the type of movement you want. A transfer of motion indicates a set of basic machines to either increase mechanical efficiecy or produce a wholly new type of movement.



More Great Illustrations



Simple Machines
Simple machines are useful because they can make a physical job easier by changing the magnitude or the direction of the force exerted to do work.

Let's take a common example. Have you ever tried to unscrew a nut, bolt, or screw from something with you bare hands and discovered that it was just too tight to loosen even if you had a good grip? So what did you do? You got the proper tool, such as a screw driver or wrench, and unscrewed it! Why is it that it's so easy to unscrew with a tool when you can't with your bare hands? Well, in reality the wrench and screw driver are examples of a wheel and axle, where the screw or bolt is the axle and the handle is the wheel. The tool makes the job easier by changing the magnitude of the force you exert.

Pulley - A single pulley simply reverses the direction of a force. When two or more pulleys are connected together, they permit a heavy load to be lifted with less force. The trade-off is that the end of the rope must move a greater distance than the load.

Wheel and Axle - In this machine a wheel or spoke is locked to a central axle so that when one is turned the other must turn. A longer motion at the edge of the wheel is converted to a shorter more powerful motion at the axle. In reverse, a short powerful force at the axle will move the wheel's edge a greater distance.

Lever - A lever is a stiff rod that rotates around a pivot point. Downward motion at one end results in upward motion at the other end. Depending on where the pivot point is located, a lever can multiply either the force applied or the distance over which the force is applied.

Inclined Plane - This simple machine allows for an object to be moved vertically without being lifted.

Wedge - A wedge converts motion in one direction into a splitting motion that acts at right angles to the blade. Nearly all cutting machines use the wedge. A lifting machine may use a wedge to get under a load.

Screw - A screw is a central core with a thread or groove wrapped around it to form a helix. While turning, a screw converts a rotary motion into a forward or backward motion.

Gears and Chain

Gears do a few things. They make the number of revolutions applied to one gear greater or lesser than the number of revolutions of another gear that this is connected to. They also increase or decrese power depending on how they are oriented. They also simply transfer force from one place to another.

Gears and Gear Reduction
Gears are round levers. Example: Small gear is attached to a motor. Small gears moves at 1000 rpm. The small gear engages a large gear. The large gear turns one half a rotation of every rotation of the small gear equaling 500 rpm. This ratio is said to be 2:1.

Decreasing th speed this way increases the torque. Power out is now twice the input. Some power is lost due to friction in this transfer of power. Same size gears will only transfer motion and not effect speed or torque.

Speed decreases when going from small to large gear.
Speed increased when going from large to small gear.

Using Motors with Gear Reduction
Highest efficency in gear reuction is found when a driver gear turns a hub gear, which in turn had a permanent smaller gear attached to the same shaft. This is how our geared motors work. So the offset shaft.

Our Motors:
Reduction = 50:1 metal gear

Anatomy of a Gear
All gears have teeth. The orientation of these teeth determine their funtion and style.

Spur Gears - We will mostly use spur gears. The teeth are situated on the outside edges.




Bevel Gears - Teeth are on the face of the gear. They transmit force perpendicularly.



Miter Gears - Like Bevel gears move force at 90 degrees.





Relatives
Worm Gear is a combination of a gear meshed with the threads of a screw. This combination changes the direction of turning motion by ninety degrees. Worm gears also decrease the speed of turning from screw to gear and increase its force.



Rack and Pinion - A single gear, the pinion, meshes with a sliding toothed rack. This combination converts rotary motion to back and forth motion. Windshield wipers in cars are powered by a rack and pinion mechanism. A small pinion at the base of the wiper meshes with a sliding rack below.



Cam is a wheel with shaped bumps on it. Cams are often connected to rods, levers, or springs. The bumps on the turning cam push down on the end of the lever making it raise an element again and again.

Crank and Rod - The crank is a wheel with a pivoting arm attached near its edge. The arm is attached by a hinge to a rod. When the crank turns, the rod is pushed back and forth. Alternatively, if the rod is pushed back and forth at the right speed, the crank will turn. The crank and rod shown here are part of giant steam engine.




Gear Anatomy
The size of a gear is expressed at pitch. Calculated by counting teeth and dividing by the diameter of the gear. Common pitches include 12, 24, 32 and 48.

The degree of slope on the face of each tooth is called the pressure angle. Most commonly 20 degrees.

Pulley and Belts/ Sprockets and Roller Chain
Akin to gears are pulleys and spockets. Thes are two geared wheels that transfer motion using a belt or chain.

Both Pulleys and Sprockets allow for more slop in the construction. Not as precise as gears. Actually without slack or slop the system will not work.

Pulleys come in Cog and V type varieties. The Cog type reqires a precicely matching belt. We have samples in the lab.

Motor Shaft Connections
Attaching elements to the motor shaft is a basic design consideration.
Many of the sprockets that we have set screws built into them.

Otherwise you will have to build a shaft coupler. This is an adapter that fits the motor bore on one end and your work shaft on the other.

Pull and Push
Both pull and push are available as standard. The pull action is converted to a push action by fitting a suitable thrust pin




Force / Stroke
When a solenoid is fully opened it has a large air gap. The reluctance of this air gap keeps the magnetic field small and the force correspondingly low. As the plunger closes, the reluctance falls and the magnetic field increases. For this reason, the force obtainable from a solenoid increases as the plunger closes.

The tubular solenoid is designed for longer strokes and higher forces than standard solenoids. This is due to the coil being fully enclosed with the steel casing so minimising magnetic flux leakage and maximising the magnetic field.

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