January Exam Flashcards
Two categories of bearing for the two types of mechanical motion
Three types of mechanical contact bearings
Three types of mechanical non-contact bearings
Linear, rotary
Sliding, flexing, rolling
Fluid film (hydrostatic)
Fulid film (hydrodynamic)
Two types of friction, equations for both
Static friction, kinetic/dynamic friction Fs = μ*F(N) μ = coefficient of static friction Fk = μ*F(N) μ = coefficient of kinetic friction F(N) = force downwards (m*g) Fs/Fk = force required to move object
Slide bearings: used for linear or rotary?
Where are they placed between?
Pros, cons
Both
Between shafts and housings
Pros: low noise, cheap, small
Cons: high friction, can be damaged from lubricants, stringent lubricant requirements
Special slide bearings: jewel bearings
Properties
Where are they used?
Low friction Thermally stable High hardness Bearing is stronger than shaft Watches, compasses, precision instruments
Sliding bearings in a linear application: pros, 5 of them
Self lubricating Low friction, noise Dust and shit doesn't stick to bearing Resists corrosion Light weight
Sliding bearing: dovetail linear slide
Pros, cons
Pros: adjustable tension
Precision defined by adjustment
Cons: regular maintenance
Undefined static friction
Application loading: what is is
Two types, which direction on a shaft they come from
Direction of forces on the bearings
Axial/thrust load: goes through shaft
Radial load: perpendicular to shaft
Rolling bearings: two types
What two grouping types for each of these are there
Spherical roller thrust bearings, tapered roller bearings: pros and cons
Ball or roller Can be classified as either radial or thrust/axial SRTB: Pros-help with misalignment Cons-expensive to produce TRB: Pros-cheaper to produce Cons-Doesn't help with misalignment
Needle radial bearings: applications and pros
Planetary gears
Universal joints
Constant mesh gears
Pros: low profile, lightweight, higher load capacity, cheap
Linear roller bearing-where sliding and rolling meet
Pros, cons
Pros: reduces risk of ‘Stick-slip’ (increased static friction due to angular forces)
Cons: expensive, bigger, more shaft damage
Slides: whats the ratio of length to width
Where should the F(pull/push) be on the slide?
1.6:1
As close to the centre line of mass as possible
Stepper motor equations: Whats pitch (definition)?
How to read a thread metric e.g. Tr, 12 x 3
Equation for number of motor steps?
P: distance between threads on a screw
Diameter of 12mm, pitch of 3mm
No. of steps = 306/( Step Angle) * Distance/Pitch
Stepper motor equations:
Relationship between travel time and step frequency
Equation for resolution
Travel Time = 1/Step Frequency
R = (Step angle*pitch)/360
Use for lead screw linear drives-think about stuff from lectures
For one revolution of a pulley, calculate distance travelled
Syringe driver, scissor action lift
Distance = Pitch * Number of teeth
Rack and Pinion: what is it, pros over belt
A rigid belt with a cog on an axel rotating along it
Pros: rigid, precise, easy to expand
Micro-stepping the stepper motor vs adding a gear box to the drive: pros and cons
MS: Pros-better resolution (256x)
Cons-torque, speed would decrease
GB: Pros-more torque, resolution would increase by gear box ratio
Cons-speed would decrease
Derailleur on a bike: what three tasks does it perform (the thing that sits on gears)
Controls chain/gear alignment
Limits range of lateral motion
Maintains a relatively constant tension on chain
Concentric and eccentric bearings: where is the shaft in both?
How to work out total offset using ‘e’ through 360°
CB: shaft at centre of the bearing
EB: shaft offset by value ‘e’, the eccentricity value
2*e
Eccentric bearings pros, cons
Pros: Can adjust for tension and remove gaps
Can adjust for wear and to counteract variance
Achieve higher motion accuracy
Cons: more expensive, set up required
Universal joint, cardan joint:
Use for both
What do they look like
What about velocity do you have to remember?
Change the axis angle of a rotational system-not for 90°
Pipe with a hinge in it
Pipe with two hinges in it
Velocity of output shaft isn’t same as input shaft unless they are aligned
Whats a Rzeppa joint used for
What effect does high and low belt tension have?
When an angle greater than 45° is needed
HBT: premature bearing wear
LBT: reduced accuracy and/or dislocation
Two ways to adjust a belt/pulley system
Adjustment distance for both
Linear and rotational tension adjustment L: slot distance R: 2*R(p)*sin(θ/2) R(p) = radius from pivot point θ = angle to move
Tension measurement methods: two of them
Force/displacement measurement
Sonic measurement
Rotary to linear equations-remember mechanics
Revise-lecture 5
When does vibration occur?
Relate it back to bearings
Equation for centrafugal force F(CF)
When centre of mass does not coincide with the axis of rotation
I.e. when there is eccentricity ‘e’ betwwen CoM and AoR
F(CF) = meω^2
m = mass
e = eccentricity
ω = speed of rotation
Effects of vibration-mostly negative but two positive
Cons: increased bearing wear, noise, heat
Material fatigue, decreased resolution
Pros: physical feedback, controlled agitation (like pile drivers)
Absolute vs relative position on a computer system
Also works for angles
AP: measure each point from an absolute origin
RP: Make a point on the shape a temporary origin and measure from there
Origin of drawing space in G code
Left and lower most location
Mechanical switch overview:
Poles
Throws
Poles (P): how many switches are activated by the same mechanism
Single (S), Double (D), 3, 4 etc
Throws (T): How many contacts can it be switched to
SIngle(S)(1): One possible, one normally open contact (NO) OR one normally closed contact (NC)
Double(D)(2): two possible, one NO AND one NC
3, 4 etc throws: one NC, rest NO
Mechanical switch overview: maintained, momentary
What do their circuit diagrams look like
Mech: throw position is maintained when switched (light switch)
Normal switch
Moment: throw position reverts to a static position when released (button)
Switch with a line pressing down into it with bar above
What is a snap-action mechanism (long definition)
Properties
Mechanism where the moving contact quickly moves from one fixed contact to another fixed contact with minimal relation to the speed the switch operates at
-Little force need to activate them
-Repeatable, precise
Over 1 million operation cycles
Other position detection devices: magnetic reed switch
What are they
Pros, cons
Reeds made from ferrous metals so that when magnetic field is applied, reeds are polarised and attract each other
Pros: sealed so can be used in specialist environments
Very small, cheap
Cons: not precisely predictable, depends on strength of magnetic field
Given resolution of system, how many steps must X/Y motors take to get to a point P
X steps = P(X)/resolution
Y steps = P(Y)/resolution
Three types of rotary incremental encoders (think about types of PC storage)
Three things they all do
Optical, magnetic, capacitive They: -inform that a step has taken place -inform about direction of travel -inform when you have reached a full rotation
Two types of optical encoders
Equation for angular distance, S(D), between ‘A’ and ‘B’ sensors
Transmissive, reflective
S(D) = (360/No. of windows) * (N + 0.25)
N = number of whole windows between A and B
Optical encoding: 3 types of encoding to do with channel A and B (imagine a square voltage wave)
X1: Channel A OR B produce a pulse every 4° i.e. one +ve edge
X2: Channel A OR B produce two edges (+ve and -ve) every 4° i.e. an edge every 2°
X4: Channel A AND B produce four edges (+ve and -ve) every 4° i.e. an edge every 1°
Whats PPR
Given PPR and X1/2/4, whats the resolution of the encoder?
Given PPR and period of pulse output, whats rpm
Pulses Per Revolution 360/(PPR/X) X = 1,2,4 depending on X1/2/4 Frequency = 1/Pulse period rpm = 60*(Frequency/PPR)
Pros of linear incremental encoders in linear drive systems-three points
If there any ‘backlash’, it won’t be detected by rotary encoders
Instant response to linear motion
Very high resolution up to 1 nm
Absolute encoders: properties, pros, cons
Rather than a single track, they have multiple tracks with encoded data
Pros: needed for precision positioning
Can be optical, inductive, magnetic or capacitive
Cons: expensive
Absolute encoders: whats gray code
Why is it like this?
Binary sequence where only 1 bit changes at a time
So there is no confusion during position transition
Whats the definition of a servo?
What frequency and type of signal do servos generally need?
An automatic device that uses error-sensing, negative feedback to correct the action of a mechinism
Uses built-in encoder
50 Hz, PWM
Servo: what is duty cycle
What four things make up a servo?
Percentage of time that signal is high relative to period of wave
DC motor, gear box, potentiometer, control circuit
Analog vs digital servos:
What is the dead band?
Difference between analog and digital
Amount by which the pulse must vary before any change in position happens
Analog has large dead band
Digital has smaller dead band and better torque and response time
Analog cheap, digital expensive
What is servo torque and speed dependent on?
What are they measured in?
Voltage applied
kg.cm, rpm OR stupid imperial
Whats a solenoid?
Why are springs used with solenoids?
What component do you need in parallel with a solenoid and why
A cylindrical coil of wire acting as a magnet when carrying an electric current
To get the return action required
Diode, cos of back EMF
Why use relays?
4 points
Why use them for the last application
Circuit isolation Power switching Signal detection Logic systems Reliable with stuff like radiation
What is the definition of:
hydraulics
pneumatics
Tech concerned with conveyance of liquids through pipes and channels
Tech concerned with mechanical properties of gases
Hydraulics: wheres the fluid held
Pros, cons
In a closed system
Pros: Can exert big pressure as fluid can’t be squashed
Needs less energy
Self lubing
Cons: fluid is a contaminant so sometimes can’t be used
Pneumatics: pros, cons
Pros: cleaner than hydraulics
Faster
Cons: needs a compressed source of gas
Air can contain water which can cause problems
Hydraulic pump: why types of energy does it convert from and to
2 steps for how it works
Converts mechanical to hydraulic energy
- Mech action creates a vacuum
- Pressure forces liquid from reservoir into system
Positive-displacement pump, whats it do?
How to make a hand pumped jack easier to use?
Displaces same amount of liquid for each pumping cycle
Use a longer handle
What is cracking pressure in relation to valves?
What valve does every hydraulic system need?
Pressure at which the valve will open
Pressure relief valve
Cylinder equations:
Pressure
Force: port A to port B
Port B to port A
Pressure (psi) = Force(lb)/Surface area(in^2) F(A) = Ps*π*(Cd/2)^2 Ps = pressure F(B) = PS*π*(Cd^2-Rd^2)/4 Cd=cylinder diameter Rd=rod diameter
Given volume of fluid V in cylinder, how far does the piston travel?
Moving from port A to port B
Moving from port B to port A
P(TD) = 4V/π*Cd^2
R(TD) = 4V/π*(Cd^2-Rd^2)
Pistons in parallel vs series: which order do they rise in when a weight is on one of them and by how much
Parallel: Non weight, then weight
Both rise by same amount
Series: Rise at same time
Non weight rises less
What happens when air is compressed, what does this do
Two types of pneumatic components
Moisture condenses inside tank, must be removed or may corrode
Dry air, oil lubricated
