Machine Tools/gears Flashcards
A ____ is a disc, wheel or cone that has drive teeth that are meshed, or interlocked, with a mating gear
Gear
A ___ is a straight gear or a gear without a curve. Combined with a gear it converts between rotary and linear motion
Rack
5 inherent disadvantages of gears when compared to belt and chain power transmissions
-expensive
-relatively heavy
-require more exact center distance
-require more maintenance (ie backlash, lube, alignment
-has no inherent overload protection
4 general categories of gears
-spur
-helixal
-bevel
-worm
Considered to be the most basic a of gears and are most likely to be produced on general machine shop equipment
External spur gear
The teeth on an ____ are arranged around the inside dia of a hollow cylinder. Although more difficult to cut, the teeth have more contact area and the shafts can be mich closer together
Internal spur gear
Teeth on a ___ are cut at an angle to the gear axis, in a shape of a helicopter or 3 dimensional curve like a spring
Helical gear
Helical gears can operate at higher speeds with less vibration. Able to transmit larger loads than spur gears of the same diameter. T or f?
True
The teeth on a LH helical gear slope down to the left and RH slope down to the right. T or f?
True
Teeth of ___ are similar to the teeth of spur and helical gears, buy theu are formed on a surface of a cone instead of a cylinder.
Bevel gears
Bevel gears are used to transmit power between intersecting shafts, this allows the transmission of power around corners. T or f
True
Bevel gears that meet at 90 and have the same # of teeth are called
Mitre gears
Bevel gears that meet at ant angle other than 90 are called
Angular bevel gear
4 styles of bevel gears
Straight
Spiral
Zero
Hypoid
A _____ drive consists if a special threaded driver called a worm that drives a gear called worm gear or worm wheel
Worm gear
Worm gear drives connect crossing shafts at 90’ and are always used as reduction units, t or f?
True
The line of action on 2 gears that are in mesh crosses a line tangent to the pitch circles of 2bgears at an angle forming the ____?
Pressure angle
Most common pressure angles in use today are…?
14.5, 20 and 25deg
This pressure angle are sometimes described as obsolete but can still be found in lots of older equipment
14.5deg
This pressure angle hear tend to he the most popular choice for new designs
20deg
This pressure angle gears are generally used where increased strength or less undercutting is required
25deg
Minimum # of teeth in 3 pressure angles
14.5deg = 32min teeth
20deg= 18
25deg= 12
A ___ is a series of 2 or more gears in mesh
Gear train
3 types of gears in a gear train
-driver gear
-driven gear
-idler gear
A ___ imparts power to the drive train, It is the first gear in a simple gear train
Driver gear
A __gear receives power from a driver gear. It is the last gear in a simple gear train and is connected to the driven shaft
Driven gear
___ gear lie between the driver and driven year and have no effect on the ratio of a gear train
Idler gear
_____ have 1 driver, 1 driven gear and they may or may not include idler gear. All gears are arranged in the same plane. Also referred to as single reduction trains
Simple gear trains
____ have atleast 2 drive gears and 2 driven gears and may or.may not include idler. Atleast one pair of gears are locked together and rotate on a common shaft
Compound gear trains
As a general rule, if the # of shafts is odd, the first and last gear rotate in the same direction, and vise versa if the shafts are an even number. T or f
True
The rate of change between one shaft and another is expressed as the ___?
VR or velocity ratio
The torque ratio from one shaft to another is called ___, and is identical to the velocity ratio
MA or mechanical advantage
If the VR is greater than 1 a speed reduction drive is described. If the VR is less than 1, a speed increaser is described, t or f?
True
3 configurations of spur gear
-internal
-external
-rack and pinion
Difference between spur gears and helical gears
Helical gears are spur gears that have teeth cut at an angle
The ___ of a gear is the distance from a point on one tooth to the pitch to a corresponding point on the next tooth, measured on the pitch circle
Circular pitch
___ is the radial distance from the root of the gear tooth to the pitch circle
Dedendum
The ___ of a gear tooth is the contact or working surface of the gear tooth between the pitch circle and top of the tooth
Face
The ___ of a gear tooth is the surface of the tooth that lies between the pitch circle and root circle of the gear
Flank
The radial distance between the outside diameter and the root diameter
Full or whole depth
The ___ of a gear is the diameter of the outside circle
Outside diameter
The ___ of a gear is the effective diameter of the gear where all ratio and speed calculations are made
Pitch diameter
The ___ of a gear is the circle that passes through the roots of the gear teeth. It represents the minor diamter
Root circle
The __ of a gear tooth is the length of the arc that describes the thickness of the gear tooth along the pitch circle
Circular thickness
As the diametral pitch of the gear increases, the size of the gear tooth decreases, t or f?
True
The linear pitch of a rack is equal to the circular pitch of a mating gear, t or f?
True
The ___ is the radial distance that a gear tooth extends into the space of a mating gear at full engagement. It is equal to 2x addendum
Working depth
The clearance for a particular gear is often adjusted depending on the method of manufacturing, and it is usually between ?
0.157/DP AND 0.250/DP
Most spur gear teeth are made in a shape of an ___ curve
Involute
For spur gears to mesh correctly they must have the same ___ and ____
Diametral pitch and pressure angle
For metric gears, the ___ of a gear describes the size of a gear tooth. It is equal to the pitch diameter of the gear divided by the # of teeth on the gear
Module
The teeth on a ___ are arranged on the face of a cone, and they taper from one end to the other. They transmit motion between now parallel search and usually intersect at 90
Bevel gears
The teeth on ___ are straight and point directly at the apex of the cone, they taper from one end to the other and have an involute tooth form. They have 20’ pressure angle. Should not be used at peripheral speeds higher than 1000fpm
Straight bevel gears
The teeth on ___ are curved and set at an angle to the axis of the gear. Depth of tooth can be parallel or tapered. Smoother and quieter than straight. Can rotate at speeds higher than 8000fpm
Spiral bevel gear
___ bevel gears incorporate the best features of a straight and spiral bevel gear. Teeth are curved like a spiral gear but they have no spiral angle. Useful in precise applications in which backlash must be kept to a minimum. Should not be used higher than 1000fpm
Zerol bevel gear
___ bevel gears differ from other bevel gears in that their shafts do not intersect, but are offset instead. The most difficult of bevel gears to make but offers the most advantage that more than compensate their higher cost (strength, higher ratios, sliding action, mounting)
Hypoid bevel gear
Contact between bevel gears occurs on a cone called the__. The crest of the teeth for the ___, and the roots of the teeth form the ___?
Pitch cone
Face cone
Root cone
The angle between the pitch cone and the axis of the gear is caller?
Pitch angle
All gears are defined by these 3 basic elements, these are?
-number of teeth
-size of tooth (DP,CP or module)
-pressure angle
___ uses a form cutter to cut away the metal between gear teeth. Shape of tooth depends on shape of cutter. Less accurate than generating. Best suited for 1 offs and repair work
Form cutting
___ relies on the relative motion of the gear blank and the cutter to shape the tooth. More accurate way to cut a gear but requires specialized and expensive machinery. Proffered method when cutting large number of gears or for gears with demanding applications
Generating
Gears are milled with __. These are arbor mounted form cutters shaped to cut the space between gear teeth
Involute gear cutters
What depth of cut when milling a spur gear after all the set up is done?
To the whole depth of the gear
Set up for milling a helical gear on a manual is same as for a spur gear but with the following 4 differences
-blank must be larger
-table screw must be geared to the dividing head
-table must be swiveled to the helix angle of the gear
-different involute cutter used
This is the distance from one tooth to the next, measured at right angles to the face of a tooth. For helical gears.
Normal circular pitch
The distance the table travels in 40 turns of the crank, assuming a 1:1 gear ratio
Lead of the table
This is how far the gear must travel to rotate once
Lead of the workpiece
The table must be swiveled to match the helix angle of the gear, otherwise the profile of gear tooth is wrong. Swivel clockwise when cutting LH helix and CCW when cutting RH helix, t or f?
True
Milling a rack presents 2 challenges
-axis of the cutter must be aligned with table axis
- table must be advanced by an equal amount to the linear pitch
A ___ re-orients the axis of the cutter by 90deg. This allows the machine to cut a rack that is as long as the table travel
Rack milling attachment
The easiest way to advance the table is with a ___?
Digital readout (DRO)
Is it possible to mill a bevel gear? Yes or no
Yes, but a milled bevel gear is inaccurate and noisy. Should be the last resort
Is it possible to mill a bevel gear with curved teeth? Y or n
No
2 main generating processes
Hobbing and shaping
___is a fast and accurate way to cut a spur, helical and worm gears. The tooth form is generated by the relative motion of the cutter and the blank
Hobbing
When hobbing, the cutter is called?
The hob and is performed in a hobbing machine
A gear hob cannot cut internal gears or gears sitting next to a shoulder, t or f?
True
A ___ uses reciprocating motion to cut a gear. The cutting tool is shaped like a pinion with the desired tooth size and pressure angle
A gear shaper
Shaping is slower than hobbing, but it allows the cutting of internal gears, herringbone gears and gears sitting next to a shoulder, t or f
True
Mass produced spur and helical gears are often ___?
Rolled
Rolling is a cold forming process. The teeth are stronger as a result, and the surfaces of the teeth become work hardened. Usually needs less subsequent finishing or heat treatment, t or f
True
High performance gears are usually hardened and then finish-ground to a precise shape. Useful for gears harder than RC50. T or f?
True
The purpose of shaving is to improve the shape and surface of gear teeth. Just as good as grinding but is limited to gears no harder than RC30, t or f?
True
3 common ways to measure a gear are by
-using a gear tooth vernier
-measuring over wires
-measuring across a number of teeth with a caliper (chordal measurement)
Most gear sets require clearance in the form of ___. It is thr amount by which the tooth space exceeds the thickness of the tooth at the pitch circle
Backlash
3 ways of measuring backlash are
-feeler gauge
-dial indicator
-soft wire
Free hobbing works best with worm gears. Results for spur and helical gears are unsatisfactory and these gears are relatively easy to cut using standard form cutters. T or f
True
Before free hobbing a gear blank, it must be
Gashed
4 Advantages of helical gear compared to spur gear
-operate more quietly with less vibration
-can transmit more power
-able to operate at higher speed
-center distances may be adjusted to suit design requirements
3 Disadvantage of helical gears
-more complex and expensive to manufacture
-transmits power less efficiently and generates more heat compared to spur gear
-they generate axial thrust
A ___ transmits motion between non-intersecting shafts at 90deg.
Worm drive
5 advantages of a worm drive over other types of gearing
-high torque
-high speed ratio in small places
-self locking when lead angle is less than 5deg
-accurate rotary positioning
-very quiet
Worm gearing is widely used for winches, Jack’s, conveyor belt drive because of its ability to transmit high torque at low speed, also has self locking ability. T or f
True
3 types of worm thread in common use are..
-coarse pitch
-fine pitch
-brown and sharp
This type of worm threads are intended for light duty applications in which the accurate transmission of motion is the most important configuration. Has a pressure angle of 20deg, size of thread ranging from .030 to 0.160in
Fine pitch worm thread
This type of worm threads are intended for heavy duty industrial applications in which transmission of torque requires a strong gear set. Typically has a pressure angle of 20deg, but sometimes has 25deg when greater strength is needed
Coarse pitch thread
This worm threads has a 14.5 pressure angle, looks like acme thread. Are obsolete and are only found in older machinery
Brown and sharp worm thread
3 types of contact between worm and worm gear are
-plain
-single enveloping
-double enveloping
___ worm gearing is the simplest set up and consist of nothing more than a worm in mesh with a helical gear. Easy to make but lacks strength of other types due to contact only at 1 point
Plain worm gearing
This type of worm gearing consists of a worm in mesh with a throated worm gear. A throated worm gear has teeth that wrap around or envelop the worm. Stronger than plain worm gearing
Single enveloping worm gear
The best form of worm gearing. Consists of a throated worm in mesh with a throated worm gear. Each element wraps around and envelops each other. This type of gear worm is very strong
Double enveloping worm gearing
4 methods of mass producing worms
-milling
-generating
-rolling
-grinding
___ , also called axial pitch, is the distance from a point on one thread to the same point on the next thread. Must be same as the circular pitch of the mating worm gear
Linear pitch
Multiple start threads offer these 3 advantages
Larger minor diameter
Larger load bearing area
More efficient transmission of motion
4 ways of spacing the starts when cutting a multiple start thread on a conventional lathe are by
-moving compound rest (most convenient)
-indexing the back gear
-indexing the workpiece using a drive plate
-using the thread chasing dial
Best material to use for a worm drive when long life is important
Hardened steel worm and bronze worm gear
What type of shafts do hypoid gears connect
NoN parallel and non intersecting
