Notes on Mechanical Devices for the Electronics Experimenter

Summary

This is the title of a book. I got this book because I don't know much about mechanical engineering and this seemed like a great intro.

The introduction says it's meant for amateur experimenters to get a good foundation for experimentation and measurement and build by designing<=>building<=>testing.

Perfect!

Ch 1: Basic mechanical principles

Fundamental Qualities

We need to define some fundamentals before getting to the interesting stuff. Let's power through it.

There's 3 fundamental qualities that makes up reality, physics-wise:

1. Length - was once defined by a metal rod. Now it's defined by the speed of light.

2. Time - Once defined by a fraction of a day in 1900. Now it's defined by periods of radiation from a cesium atom.

3. Mass/Weight - Mass is constant, but weight is "mass * the-acceleration-of-gravity". In any case, for us it's the same.

Notably, here's a few units of mass:

• kilogram - kg -- 2.204 pound-mass
• pound-mass - lbm -- 0.4536 kg
• slug - slug -- 32.174 pound-mass

Yes, we're just treating pounds as a mass by calling it pound-mass.

There's also these other relevant units of reality:

• Electric Current - force generated between two conductors at some length, measured in "ampere"
• Luminous Intensity - light-ness, measured in "candela"
• Temperature - measured in "kelvin" as well as F and C.

Vectors & Motion

How do measure how stuff moves across space (length) and time?

Vectors

Vectors show you force has a horizontal and vertical component. For example, if this slash "\" had an arrow pointing down, then if it were moving a box, that would push it both into the floor and across the floor. Woo hoo.

Motion

Here's some key things:

1. Speed / Velocity - distance over time

2. Rotational Speed - revolutions per minute or angular distance traveled (degrees per second, radians per minute)

3. Acceleration - the rate of change of velocity. So if acceleration is 0, then the velocity is constant. (d*sec^-2)

Force, Work, Power

Wow, I can't believe these common-sense definitions. So much clarity compared with high school:

1. Force - the tendency to cause motion, or rather to cause acceleration (this is like mass, an absolute unchanging thing) -- newton

2. Work - force as applied through a distance (this is like weight, force that becomes tangible in-action) -- force * distance

3. Power - the rate of doing work or producing energy. (this is like velocity, work over time) -- work/second

3.1 Torque - the turning effort of a wheel. (this is measured in the same stuff as power, but uses horsepower, watts, or foot-pounds*minute^-1)

Force makes sense because whenever you move something, you're causing it accelerate. And this is different from work, which is like "over how much distance (rather than how much time) was force applied? Force is a vector and work is more of an outcome.

Also what blows my mind is they have this picture of a box hanging by two suspension wires, where one is horizontal, and the other is on the right corner at a 30 degree angle upwards. The 3rd force is the vector of gravity. So based on how heavy gravity happens to be, it changes the amount of force exerted by the horizontal vector because the angled suspension wire also applies a force through that trigonometry cosine math.

work IS energy: foot-lbs is the same as joules, kilowatt-hours, newton-meters, ounce-inches. Just different forms of the same thing.

Consistent Units for F = ma

System | Force | Mass | Acceleration mks (SI) | newton (N) | kilogram (kg) | m * s^-2 cgs | dyne (dyn) | gram (g) | cm * s^-2 engineering | pound (lb) | slug | ft * s^-2

Friction

In real life, friction stops F = m*a from being totally true. There's 2 important types of friction and some related concepts:

• Force Max (of friction) - the amount of force necessary to move an object bc it is greater than the friction of Force Max
• Force Normal - the amount of force pushing an object in the floor
• Coefficient u Static - a multiplier that lets you show the relationship between Force Normal & Force Max such that F(max)= u(static)*F(normal)
• Force Friction - this is a changing amount, because there is less friction for an object already in motion
• Force Dynamic - the amount of friction force on a moving object
• Coefficient u Dynamic - same as Static, but for a moving object

Levers

Levers have 3 magical features:

• Change direction of force -- A downward force becomes an upward force. If you have a spring, an upward force becomes a downward force.
• Amplify force -- When the load is closer to the fulcrum than the effort is, then more force is applied to the load!
• Amplify motion -- When the effort is closer to the fulcrum than the load is, then the distance moved by the load is greater than the effort!!

Holy Shit!

There are 3 types of levers:

• where o is the load, x is the effort, and A is the fulcrum, and ______ is the arms of the lever, distinguished as a load arm and effort arm.
• where effort force is applied against the arms
• where load is lifted up in first, second, and third class levers

• First Class - classic

o______x

- - - A - - -

• Second Class - specializing in amplifying force, because the load is closer to the fulcrum than the effort, by definition!

________

A - - o - - x

• Third Class - specializing in amplifying motion, because the effort is closer to the fulcrum

________

A - - x - - o

Wheels: the same as levers

A wheel is a Type 3 Lever! If you have a wheel, with an axel (a smaller circle, inside the wheel, on which force is applied, like a bicycle, generally), then the axel-circle is where the effort is applied, and the wheel gets tangential force applied as the load, and its motion is amplified!

Ohh my goodness!

Pulleys

Not sure if you've looked at a pulley closely before, but it has something called a "block", "sheave," "shaft," and "housing".

• TODO: add picture*

What I didn't know is that with a pulley system, the load you are lifting is connected to the block itself, and the pulley that's lifting the load is lifted up at the same time as the load. There's also the anchor pulley that stays in place as expected. It's because the rope is rounding the sheave that the pulley gets it's mechanical advantage of +1 for every strand supporting the load.

Inclined planes and screws

Inclined Planes

If it's hard to lift something, use an inclined plane. Theoretically speaking, you should get quite the mechanical advantage through this, though friction makes that not the case. However here's how it goes ideally:

A triangle: x=4, y=3, z=5, where x is the length, y is the height, and z is the hypotenuse.

• TODO: add picture*

The theoretical mechanical advantage is: 5/3. I guess if there was no friction in the world, it would take nearly twice as less effort to push a box up stairs vs lifting it up from the ground to your hips. Not sure if this applies only to the vertical distance or to work or w/e. I'm a little unclear on exactly what "mechanical advantage" means.

• TODO: clarify "mechanical advantage"*

Screws

A screw is an inclined plane wrapped around a cylinder.

planetary devastation!

There's a way to translate "rotary" motion into linear motion using a screw, nut, and threaded rod. Here's what you do.

• Let's say you had a nut fixed in place. Then when you rotate a screw, the screw moves linearly through the nut hole.
• But instead, let's say the screw is fixed in place. AND the nut has a hole in it where a steel thread is roped through this hole and made taut, as if the nut was hanging like a shower curtain on a shower rod. THEN when the screw turns, that motion is transferred to the nut, such that it travels up or down LINEARLY along the screw.

• TODO: add picture*

Overview of Chapters not-relevant to building lasers: 2-6

I did not read these chapters carefully but only skimmed them for a sense of what is important to robot builders and to familiarize myself with more mechanical things and also to know where I might look if I needed to know these things in the future.

Here's the chapter list:

(#2). Sensors and control

(#3). Motors

(#4). Motor control

(#5). Stepper motors

(#6). Solenoids

Sensors and control

There's open-loop control and closed-loop control.

Open-loop control: you have a theory of what will happen, and then you command the machine to act accordingly. Then it does its best.

Closed-loop control: After the machine acts, there's a measurement of what is actually happening. This feedback returns to inform the input about what the machine should actually be doing.

Position sensing: you can see how many times a gear has turned by using light and a hole in a wheel to track its revolutions, or derivative measuring systems.

Motors

A motor acts a bit like a generator, which means it generates an electric field (and therefore also a magnetic field, bc the 2 are intertwined)

Curiously, the polarity of the motor, which I believe is a magnetic property, is something that "lowers" the voltage of a motor.

Not really sure how voltage of a motor translates into its capacity to generate power or work or joules.

Did some research and you can look more info on that here: How are current and voltage related to torque and speed of a brushless motor?

Basically: V = E + IR |OR| V = IR - E depending on the motor type, where V = voltage, E = internally generated voltage generated by magnetism, I=current, R=resistance.

Voltage is the difference in charge between 2 points.

Current is the rate at which charge is flowing.

Resistance is a material’s tendency to resist the flow of charge (current).

Voltage Current Resistance and Ohm's Law

So as it turns out, torque of a motor is related to the current, so if you increase the voltage, you increase the work (torque) done by the motor.

Motor control

There's different circuit diagrams for different systems of controlling a motor.

It seems like if you have a steady voltage, and you want to adjust the current to adjust the work, the thing you control is resistance.

By using a "variable resistor" or an uninjunction transistors, you can control the resistance and therefore the current.

Not really sure if this is how it works because I did not read this chapter.

Stepper motors

When I imagine a motor, I think of it as just either running or being turned off.

Apparently, there's such a thing as a stepper motor, which has more states that it can be in while it's turned on.

Because you can control it's state while it's turned on, you have more control over the timing for when power is being generated.

There's 3 types of stepper motors:

• permanent magnet
• variable reluctance
• hybrid

Solenoids

This device transforms electrical energy into mechanical energy. Here's how it works:

• there's a hollow cylinder
• there's a coil wrapped around it
• there's a magnetic plunger that fits into the cylinder
• when you energize the coil, the plunger will be pulled into the cylinder
• then you de-energize the coil
• a spring can return the plunger to its starting place (hanging at the edge of the cylinder poking out)

• ToDO: Add picture*

Apparently these are often used in motors.

Ch 7: Gears and pulleys

Belt Drives

When you create a machine, you want 1 source of power, and then you want to distribute that power in various ways to different elements to make the system perform work.

So gears and pulleys give you more ways to not only translate the direction of power and its location, but also to amplify the power through mechanical advantage.

Here's 4 different gear/pulley systems:

• Wheel drive - wheels spin each other
• Gear drive - gears spin each other
• Belt drive - a belt connects two wheels, that spin each other
• Chain and sprocket drive - a chain connects two gears, that spin each other

There's a bunch of rules on page 85 for how to design belt drives.

There's also a diagram of how to make a belt.

Gears

There's ALOT to design about gears. Here's something that's notable to understand about why their design is so complex:

• if you have a gear A
• you must have a different design gear B for it to rotate on A
• if you have a gear C that "meshes" with gear A
• then generally speaking, gear B and gear C WILL NOT mesh together
• But... it is possible to design "tooth profiles" where all the gears can mesh with each other

Although gears lose a little power to friction, you can stack them and increase your mechanical advantage.

In a Black and Decker Model 9072 cordless power screwdriver, it's handheld but has a mechanical advantage of 81 for like-woah torque.

Keep in mind that you can have a shaft of a gear be the axel for another gear, which, when connected to another gear, has the lever thing happening.