Gearing Flashcards
For gears to mesh properly they need:
Same diametral pitch
Same pressure angle
Correct hand of gear (unless straight)
Shafts in the correct position
And?
Same helix angle
Two types of gear profiles?
Involute and straight
Describe involute tooth profile?
Most gears use involute, provides the smoothest tooth profile
Straight cut teeth are:
Noisy
Not suitable for high speed or power applications
What are some main causes of gear wear?
Lack of lubrication
Abrasives in the gear train
Incorrect tooth contact
Extended heavy loads
What can be a sign of excessive wear in gears?
Excessive backlash
What is floating gear or shaft?
When one gear or shaft is allowed to move axially to its best position. Common with double helical and herringbone gears
Why is most of the wear in gearboxes on the drive side of the tooth?
Typically most gearboxes run in one direction
On large large hypoid gears you can flip the gears to extend ________ ?
Service life of the system by flipping the gear you can transmit force onto the coast side
Simply speaking addendum is?
Top half of the tooth
Simply speaking dedendum is?
Bottom half of the tooth including clearance
Whole depth on a gear?
Addendum + dedendum
Working depth
Addendum + dedendum - clearance
Why is clearance important?
Lack of clearance will cause binding, increased friction and loss of power.
Clearance allows room for lubricant and thermal expansion
Steeper helix angles lead to smoother tooth action, but?
Have more sliding friction and greater axial thrust
What is a start on worm and wheel?
Number of thread starts on the worm.
The more the worm has the faster the worm wheel advances in one turn.
What way could you check for contact pattern?
Red lead or pursian blue with oil
What is backlash?
Clearance between gear teeth at the pitch line.
If the diameteral pitch number is small, gear teeth are physically ______?
Large
Binding can be checked by rotating gears without load.what can cause binding?
Lack of backlash or clearance between mating teeth
- incorrect centre distance between gears
- gear centre out of tolerance
- bent shaft or shaft deflection under load
- thermal expansion of the gears and the housing
Meshing teeth must have the same _______?
Pressure angle
20 degree pressure angle is stronger because it is wider at the base
For gears to mesh properly they must have:
- same diametral pitch
- same pressure angle
- correct hand of gear
- shaft in the correct position
- same helix angles
What tooth profile is smoother, involute or straight?
Involute
When even number of gears are used the last gear rotates in what direction?
Opposite of the first one
If an odd number of gears are used the last gear rotates in the _______ as the first gear
Same direction
Which direction do internal gears rotate in?
Same direction
Do idler gears count in gear ratios?
No but they change the direction of gear rotation in a gear system
Why do you need an idler gear in a gear setup?
To keep the same direction of travel
Gears that transfer power with no change in speed?
Driven sprocket has same teeth as drive sprocket. Expressed as 1:1 ratio
If the gear box is a speed reducer it is expressed as a ratio 2:1. What does that mean?
For every 2 revolutions of the input shaft there is 1 rotation of the output shaft.
How do you figure out reduction ratio?
Divide driven teeth by drive teeth.
How can you tell if it is a speed increaser or reducer? Often called overdrives
If the ratio is not a whole number
.50:1
If you increase torque you decrease _______?
Speed
1: shafts parallel to each other
2: centrelines intersecting at right angles
3: centrelines crossing at right angles
4: special cases where intersecting or crossing not at 90degrees
External spur gear
Direction of gear and pinion is different. If they need to be the same an idler must be used
Internal spur gear:
- rotate same direction
- compact
- carry same loads/speeds as external
- more teeth engagement
- quieter
Rack and pinion gear:
-Convert rotary motion to a linear motion
- rack is the flat part
Helical gears:
- similar to spur gears but except gear teeth are on an angle to the bore of the gear.
- helix angle varies from a few degrees to the standard 45 degrees
- creates a smooth quite drive
When parallel shafts are connected by a single set of helical gears the shafts develop axial thrust. The direction of the thrust depends on:
- Weather the shaft is a drive or driven
- direction of rotation
- the hand of the gear on the drive shaft
Axial thrust is compensated by?
Axial thrust bearings
Thrust of the driven shaft is always opposite to the direction of thrust of the drive shaft
Double helical gears:
- two helical gear pairs of opposite hands
- thrust generated in one of the gears is counteracted by the thrust in the other hand
Herringbone gears:
- slow turning heavy loads
- thrust is counteracted
Straight bevel gears:
- connect shafts that intersect at right angle
- smaller gear is the pinion
- larger gear is the ring gear
- straddle mount or overhung
- produce axial and radial thrust
Straight bevel gears have _____ teeth
Spiral bevel gears have ______ teeth
Zerol bevel gears have _______ teeth
Straight - run noisily create thrust
Spiral - curved create axial thrust
Curved - run smooth and quite
Mitre gears: brush saw head
- used to change shaft direction without altering the drive ratio
- need thrust bearings
Hypoid gears (rear differential)
- transmit power between shafts that cross but do not intersect
- extremely smooth running
- disadvantage is the sliding action requires EP lubricant to reduce excess friction
Worm and wheel:
- connects shafts that cross
- helical teeth around the worm shaft are called threads.
- gear ratio is obtained by dividing # of teeth on the worm gear by # of starts on the worm
What are the contact patterns in worm and wheel gears
Non throated contact - linear pitch
Throated contact - circular pitch. Forms to the worm gear
What is PIV (like variable speed belt drive)
When preloading worm and wheel bearings the shims usually go where?
Between the retaining cap and the housing
When applying shims to worm and wheel you split the required shims in half and place them under each cap. Then what?
Wipe coating of bluing onto the worm
- rotate the worm in desired direction
- check the gear to see where the worm contacts the teeth
- adjust worm by transferring shim gaskets from one cap to another
- refer to owners manual for proper specs
What hand are most worm and wheels?
Clockwise (right hand)
Single reduction worm and wheel gear boxes come in speeds of 5:1 to ______
70:1
Multiple reduction worm and wheel have two or more reductions, and may have different reduction _______?
Methods
Planetary gear reduction:
- shafts can be inline
- may be reduction, overdrive, reversing, or direct drive.
- available in 1.1:1 to 50 000:1
- made up of sun gear, ring gear, carrier which holds the pinion gears
Sun gear is the _______?
Centre
Planet (pinions) travel where?
Surround and mesh with the sun gear
Ring gear meshes with the ?
Pinion gear
What is the reaction member in a planetary system
Part that is held
If two members are locked in a planetary system what happens?
Produces direct drive
What are two ways reduction units can be mounted?
Indirect drive - V belt or chain drive
Direct mount - couple with a coupling both units bolted to a strong rigid base
When using a v belt drive with a reducer can you have extra variations in the speed?
Yes by changing the v belt sheave sizes
Reduction units are often lubricated with which method?
Splash
Excessive noise and vibration?
Unit runs hot?
Oil seals leak
Oil level drops but oil is clean
Oil is dirty with even discolouration
Oil is dirty with colour streaks or rings
Bearing failure
What are the four methods of checking mesh patterns?
-Prussian blue (not layout bluing)
- Venetian red
- black light
- pencil lead
How much bluing should be applied when checking gear mesh?
Thin film only
When checking mesh after applying bluing you drive then worm either by hand or drill while applying ______ to output shaft?
Resistance
Good mesh pattern
Lack of backlash can cause?
Noise
Overloading
Overheating of gears and bearings
Seizure and failure
Backlash is measured by restricting the rotation of one gear member, usually the pinion. And measuring the rotational movement of the worm wheel on its______
Pitch circle (halfway up the tooth)
If space is permitted a ______ should be used to check backlash. Other methods would be?
Dial indicator
Lead wire deformed between teeth
Using feeler gauges manually fitted between teeth
Checking backlash on helical gears is checked by?
Attaching lead wire to one of the gears and feeding it through the gear teeth. As the wire travels between the teeth it is squeezed into the thickness that represents the backlash.
If lead wire readings are not uniform you would check the _______ and _______
Shaft for straightness and gear bore for eccentricity
Excess backlash is an indication of?
Teeth are wearing
What is the primary cause of of gear teeth wearing?
Not fully film lubricated
How can bearing positions on the shafts and within the housings be adjusted?
Machining or shimming. The thickness and positioning of the shims adjusts the axial float of the shafts to the desired position
How is end float measured in a gear box?
Dial indicator Either can be mounted inside or outside the gearbox.
When installing new gear box you should instal with ______ “ shims under all feet to allow for downward movement when aligning.
1/8 shims
Soft foot when mounting a gear box results in?
- distortion of internal parts leads to binding heating and eventual lubrication and bearing failure
To ensure longevity of a shaft mounted gear box where should it be located?
Reducer should be mounted as close to the support bearing as possible.
What is the best method of installing a shaft mounted gear box on a shaft?
Heat in oven to about 65degrees C (150F) expansion of the hollow shaft will allows you to slide it onto the driven shaft.
What is a major hidden danger when overhauling high ratio planetary drives?
Reverse torque lockup.
What are some telltale signs that indicate the overall condition of a planetary gear box?
- stress cracks
- heat rings
- embedded metal particles
What is the purpose of an oil scraper? It what happens if it is not installed close enough to the edge of the gears?
Scrape excess oil off the gears and distribute it to the bearings via the oil dam.
If the scraper is not mounted close enough to the edge of the gears it will not scrap enough oil to lubricate the bearings.
When assembling a gear box that has oil dams why is it super important to inspect and clean all dam reservoirs and passageways leading to bearings?
A cracked scraper could lead to catastrophic failure of a broken piece fell into the gears
Two types of gear tooth pitting?
If new gears are not available what can be used to hand dress the teeth?
Tapered oilstones (abrasive files) with low viscosity light oil to remove sharp edged spots and burrs
Never use power driven rotary stones
How can you check for residual magnetism?
And what is residual magnetism?
Good quality compass
Caused by production shops not running their magnetic tables and chunks through for the time required to demagnetize the gear.
How can a welder be used to reverse residual magnetism in a gear box?
By wrapping the welding cable around the gear box then using the welder
Or
Strip down gear box and demagnetize
