Gears Flashcards
What are the two types of gear drives?
- open gear drives
- enclosed gear drives
Gear trains that are exposed to the outside environment are called:
open gear drives
What is usually used to lubricate open drive systems?
very thick oil or grease
Which system has more excessive gear wear: an open drive system or an enclosed drive system?
open drive
*particulate matter causing additional wear
Gear drives contained within a gearbox with a lubrication supply are _____________.
enclosed
What are gear sets?
matched pairs of gears manufactured and intended to mate with each other
What 2 types of gears are usually manufactured as gear sets?
- hypoid
- bevel
What can happen if a gear from a gear set is matched with a gear that wasn’t their mate?
failure can result
What is a gear train?
when two or more gears mesh and work together as a unit
When two gears having different numbers of teeth mesh together, the gear with the fewer teeth is called the ___________ gear.
pinion
True or false:
The pinion gear is rarely the drive gear.
false;
it is generally the drive gear
In bevel and hypoid gear sets, the smaller gear is called the 1.____________ gear and the larger gear is called the 2.____________ gear.
- pinion
- crown
The crown gear is also called the:
ring gear
What is the input gear on a gear train called?
drive or driver gear (power in)
What is the output gear on a gear train called?
driven gear (power out)
What is a direct drive gearbox system?
where the input speed = output speed
*transfer power, no increase or decrease of speed
A gear train with a slower output speed is called a:
speed reducer
A gear train with a faster output speed is called a:
speed increaser
What happens to the output torque when a speed reduction occurs in a speed reducer?
the toque increases due to the mechanical advantage
speed ↓ = torque ↑
What effects the amount of mechanical advantage that’s seen in a speed reducer?
the input to output ratio
- the larger the ratio (greater the speed reduction), the greater the mechanical advantage
What is an overhung gear?
A gear with support only on one side
What 2 kinds of gears are overhung loads common on?
- Bevel
- Hypoid
General practice is to keep overhung gears very close to the bearing. Why is this?
For support, as they tend to have more shaft deflection
What is a straddle mount gear?
A gear mounted between two bearings
What are internal teeth gears?
Gears that have the gear teeth on the inside diameter of the gear
What are 2 advantages of internal gears?
- Operate more smoothly than external gears
- More tooth contact vs. External tooth gears
Compound gear trains have 1._________ or more shafts and 2._________ or more gears.
- three
- four
In any gear train with more than one gear set, what does the final ratio at the output shaft equal?
all the ratios through the gear train multiplied together
If a gear train has a reduction of 4:1 on the first set, a ration of 3:1 reduction on the second set, and a reduction of 5:1 of the third set, what would the final ratio be?
4 x 3 x 5 = 60:1
What is end play on a gearbox?
when a gear or shaft is allowed to move axially
_______________ a bearing in a gearbox is accomplished by giving the bearing less than zero axially clearance.
Preloading
In a gearbox, what is the axis of rotation?
the centreline around which a gear or part moves
What are the 4 main reasons for gear wear?
- lack of lubrication
- abrasives in the gear train
- incorrect tooth contact
- extended heavy loads
True or false:
An increase in backlash can be an indication of gear wear.
true
What is a floating gear or shaft?
where one gear or shaft is allowed to move axially into a self-centred position
On helix gears, if the teeth point to the 1._________, as viewed from the shaft axis then the gear is right-hand gear and if the teeth point to the 2.__________ it is left-hand gear.
- right
- left
In a worm and worm wheel gear set, the ___________ is the smaller gear that looks like a screw thread.
worm
The __________ side of gear teeth is the side in contact with the other gear teeth while torque is applied to the gear train.
drive
What is the opposite side of the drive side on a gear?
the coast side
- side where no torque is applied during normal operation
What is the addendum of a gear?
the top half of the gear tooth
- the radial distance from the pitch circle diameter to the top land of the tooth
What is the dedendum of a gear?
the bottom half of the gear tooth included the clearance
- radial distance from the pitch circle to the bottom land of the tooth
What is the whole depth of a gear tooth?
total height of the tooth
- addendum + dedendum = total depth
What is working depth of a gear tooth?
the depth the gear extends toward the root of the mating tooth when both gears are in full mesh
- working depth = addendum + dedendum - clearance
When it comes to a gear tooth, what is clearance?
the space between the bottom land of one gear and the top land of the meshing gear
What 3 things do gear tooth clearance allow for?
- lubricant
- thermal expansion
- gear runout
What 3 things can happen if there is not enough clearance in a gear tooth?
- binding
- increased friction
- loss of power
What is the outside gear diameter of a gear?
the diameter of the gear when measured from the top land of the gear teeth
What is the circular pitch of a gear?
the distance form the centre of one tooth to the centre of the next tooth at the pitch circle diameter
What is circular pitch also called?
gear pitch
What is pitch circle?
the diameter of the gear approximately in the middle of the gear teeth
Pitch circle is also known as:
pitch diameter
- most often known as pitch cirlce diameter (PCD)
On a gear, what is the face of the tooth?
the contact part of the tooth on the addendum or top half of the tooth
What is the tooth flank of a gear?
the contact part of the tooth below the pitch circle, including the fillet
What is the face width of a gear?
the distance from one side of the tooth to the opposite side
What is the chordal thickness of a gear tooth?
the thickness of the gear tooth at the pitch circle diameter
What is chordal thickness of a gear tooth indicative of?
tooth strength
____________ a gear tooth is cutting a slight curve on the face of the tooth from side to side.
Crowning
What is the point of crowning a gear tooth?
to reduce loads on the ends or edges of the gear tooth
- these are the loads that cause the teeth to fracture
What is the line of action of a gear tooth?
the points of contact as the gear turns
What is the helix angle of a gear tooth?
the angle of the gear tooth relative to the axis of the gear
On a worm gear, what is a start?
the number of thread starts on the worm gear
What effect does the number of starts have on the worm wheel?
the more starts, the faster the worm wheel advances in one turn of the worm
What is the involute profile of a gear tooth?
The radius on a gear tooth (across both the flank and the face) that makes the meshing of gears smoother
What indicates the number of teeth on a worm gear?
The number of starts
Number of starts = number of teeth
Where does the rolling action of gears meshing take place on the gear tooth?
On the involute profile of the flank and face of the gear tooth at the pitch circle
What is the contact pattern of a gear tooth?
the pattern on the tooth that are in contact when the gears are meshed and when checked gives an indication of relative gear position
What is the diametral pitch of a gear?
the ratio between the number of teeth on a gear and the pitch diameter
What formula is used to calculate the diametral pitch?
DP = # teeth ÷ PCD
DP = diametral pitch
# teeth = number of teeth on the gear
PCD = pitch circle diameter
If the diametral pitch of a gear is small, the teeth of the gear are physically ___________.
large
What is backlash as it pertains to a gear?
the clearance between the gear teeth at the pitch line approximately half way up the tooth face
What formula is used to calculate average backlash of a gear?
average backlash = 0.40” ÷ DP
DP = diametral pitch
What are the 3 benefits that backlash in a gear allows?
- keeps teeth from binding
- allows for thermal expansion
- allows for the lubricating film
What are the 5 common causes of binding in a gearbox?
- lack of backlash or clearance
- incorrect centre distance
- gear centre out of tolerance
- bent shafts or shaft deflection under load
- thermal expansion
What are the 2 most common gear tooth pressure angles?
- 14.5º
- 20º
Which gear tooth pressure angle is stronger?
20º
As gears mesh, the action between the teeth is 1.____________ until both teeth are at contact at the pitch line, at which time the action changes to a 2.___________ action until the teeth begin to pull out of mesh and pull apart.
- sliding
- rolling
When gears mesh, during the initial stages of contact, the 1.______________ of the tooth on one gear contacts the 2.______________ of the tooth on the opposite gear.
- dedendum
- addendum
What 5 things must gears have to mesh properly?
- same DP
- same pressure angle
- correct hand
- same helix angle
- proper shaft position
What are the 2 gear profiles?
- involute
- straight
When an even number of gears mesh with each other, the last gear rotates in the _________________ (same/opposite) direction as the first one.
opposite
When an odd number of gears mesh with each other, the last gear rotates in the __________________ (same/opposite) direction as the first one.
same
What are idler gears?
gears between the drive and driven gear
What effect do idler gears have on the gear ratio of the gears that they mesh with?
no effect on the ratio
What are 3 functions of idler gears?
- change the direction rotation (or keep it the same depending requirements)
- connect two gears together (close gaps)
- function as moveable gears for setting backlash and mesh
What type of gearbox is an overdrive gearbox?
a speed increaser
How can the overall gear ratio be calculated if only the individual gear ratios inside a gearbox are known?
by multiplying all of the ratios together
- always follow the input, as this will let you know if its a speed reducer or increaser
What formula can be used to find the mechanical advantage of a gearbox?
MA = N(out) ÷ N(in)
MA = mechanical advantage
N(out) = # of teeth on the output gear
N(in) = # of teeth on the input gear
What are the 3 basic shaft positions that allow gears to mesh properly?
- parallel
- intersecting
- crossover at 90º
What must the hand be on helical gears that are mounted on parallel shafts?
they must have the opposite hand
What two types of gears can be mounted on intersecting shafts at 90º?
- bevel (straight, sprial, Zerol)
- mitre
What must the hand be on helical gears that are mounted in a crossing over at 90º shaft arrangement?
the must have the same hand
This type of gear is cut with teeth parallel to the shaft axis and develop only radial torque and no axial thrust (unless misaligned).
spur gears
What are the 3 types of spur gears?
- external
- internal
- rack and pinion
This type of spur gear arrangement converts rotary motion into linear motion.
rack and pinion gear arrangement
What is it called when spur gears are cut on to a linear surface?
a rack
When spur gears are mounted in the gearbox housing, the correct centre-to-centre distance between the gears is half the sum of the:
pitch circle diameters of the two gears
centre-to-centre = (PCDA + PCDB) ÷ 2
True or fasle:
The torque capability of spur gears is limited to one tooth in full contact at at time.
true
What are the two disadvantages of helical gears when compared to spur gears?
- cost
- axial thrust
- friction and the required lubricants can also be considered a disadvantage
Why is it important to select the correct lubricant for gearboxes that contain helical gears?
the increased sling action increases the pressure applied to the lubricant
- EP rated lubricant is a must
What determines the direction of axial thrust of a helical gear?
the hand and the rotation of the gears
How can the axial thrust direction of a helical gear be determined if the rotation is known?
using the Right Hand and Left hand rules
- which rule to use depends on the drive gear; a right hand drive = RH rule, a left hand drive = LH rule
- to use the rule, place your right hand (or left, depending on the drive) with the fingers orientated in the direction of rotation. The thumb will indicate the direction of axial thrust
What advantage does increasing the helix angle have on the gears?
increased strength/greater torque due to more teeth being in contact at the same time
What is the main advantage of using double helical gears?
the double helical cancels out the axial thrust normally produced
Helical gears can have pressure angles from 1.______º to 2.______º, with 3._______º being the most common for industrial gears.
- 7º
- 23º
- 20º
This type of gear is similar to double-cut helical gears, but the point forms at the centre of the gear.
herringbone gears
Most bevel gears develop both axial and radial loads. What is the exception to this?
Zerol gears;
They do not develop axial loads
What can happen to overhung gears that are subject to shock loads?
the shaft deflects, causing the gear teeth to move out of proper mesh
What are the 3 types of bevel gear tooth geometry?
- straight tooth
- spiral tooth
- Zerol tooth
What is a mitre gear?
bevel gears that intersect at 90º with both gears having the same number of teeth
For spiral bevel gears to mesh, the gears must be of the __________ (same/opposite) hands.
opposite
This type of gear forms a drive between two shafts that cross over and are also at 90º to each other.
hypoid gears
This type of gear will whine or howl is they are not set up correctly.
hypoid gears
Why are hypoid gears smoother and quieter than bevel gears?
they have more sliding action
This type of gear set is used where a high ratio reduction is required in relatively little space.
worm gear set
What kind of shaft positions are worm gear sets found in?
crossover at 90º
On worm gear sets, the worm and the worm wheel always have the _______________ (opposite/same) hand.
same
The worm on a worm gear set is always the ________________ (input/output).
input
If the worm on a worm gear set has only a few starts, it tends to be self-locking. What does that mean?
the drive will not rotate backwards against the worm when power is removed
- useful for backstop applications
What types of bearings would be found in a worm gear set?
tapered roller bearing
Which type of gear set creates the most friction?
worm and worm wheel
This type of gear set is often housed in a gearbox that has fins on it.
worm and worm wheel
- the fins are used as heat sinks, due to the excessive heat generated by the gears
What 6 things affect the efficiency of a worm gearbox?
- gear ratio
- input speed
- lead angle
- gear material
- gear design
- amount of output torque
What affect does increasing the ratio of a worm gearbox have?
ratio ↑ = friction ↑ = heat ↑= efficiency ↓
What is the worm pitch of worm gear set?
the distance from one tooth to the next on the worm
- think thread pitch
What is the worm lead on a worm gear set?
the distance by which one tooth on the worm advances in one turn
What formula is used to calculate the lead of a worm gear?
pitch x number of starts = lead
What is the number of starts on a worm gear?
the number of thread beginnings machined into the worm
How is the ratio of a worm gear set determined?
ratio = number of worm wheel teeth ÷ number of starts on the worm
In a worm gearbox, the worm can be shaped two ways: __________________ is shaped like a cylinder and _________________ is shaped like an hourglass.
non-throated = cylinder
throated = hourglass
True or false:
The worm wheel on a worm gearbox can be both throated and non-throated.
true
When a worm wheel is _________________, the tops of the wheel will have flat tops.
non-throated
When a worm wheel is ________________, the tops of the wheel will be concave in the middle.
throated
What effect does whether a worm gear set is throated or non-throated have on the gearbox?
it determines the amount of torque the gears can handle
- power transfer
In a _______________ worm gear set, both the worm and worm gear are non-throated.
non-throated
In a _______________ worm gear set, either the worm wheel teeth or the worm is throated.
single-throated
- most common is throated worm wheel and non-throated worm
In a ______________ worm gear set, both the worm and worm wheel are throated.
double-throated
Which type of throat/non-throated worm gear set configuration give the maximum gear tooth contact?
double-throated
- To achieve the maximum power and life of a worm gear set, the worm wheel contact should be toward the ______________ (entering/leaving) side of the worm wheel.
- Why is this?
- leaving
- this is to allow oil to into the entering side of the gear
What are 4 heat treatments used on steel gears?
- hardened and tempered
- flame or induction hardened
- carburized and case hardened
- nitrided
What kind of application are brass gears used for?
light load or slow turning applications
What type of gears are insulators and protect against electrical current?
non-metallic gears
What are 4 advantages of non-metallic gears?
- cost savings
- quieter operation
- run with limited lubrication, if any (self-lubricating)
- absorbs vibration
What type of metallic gear is preferred for meshing with non-metallic gears?
cast iron
What are 4 common types of material used in non-metallic gears?
- nylon
- Teflon
- acetal resin
- fabric impregnated resins
What are the 3 parts of a planetary gear system?
- sun gear
- carrier with mounted planet gears
- ring gear
In a planetary gear system, the __________ gear meshes with the planetary pinion gears, which in turn meshes with the ring gear.
sun
True or false:
In a planetary gear set, the gears are always in mesh.
true
What is the reaction member of a planetary gear set?
the member of the gear set that is being held in place
- can be any of the parts (sun, planetary, ring)
What happens if two of the members of a planetary gear set are held?
the gear system becomes a direct drive system
What type of final output would be seen if none of the members of a planetary gear system were held?
it would be a neutral system
True or false:
Planetary gear sets can be either spur or helical designed gears.
true
How can a planetary gear system be made to handle more torque?
by adding more planetary pinion gears
A single reduction gearbox has 1._______ shafts and 2._______ gears.
- two
- two
A double or multiple reduction speed reducer has at least 1.__________ shafts and 2.___________ gears.
- three
- four
_______________ reducers have the input shaft and output shaft mounted on different planes.
offset
______________ reducers have the input and output shafts on the same plane.
concentric
What is the active flank of a gear tooth?
the drive side
It is common to have the mesh pattern on a unidirectional worm gear set favouring the leaving side of a worm wheel. Why is this?
to provide clearance on the entry side for lubricant to enter and form a fluid film
- on a bidirectional gearbox, it would be in the middle
What are 4 ways to check for mesh patterns in a gearbox?
- Prussian blue
- Venetian red
- black light
- pencil lead
The mesh patterns of gears should be checked and recorded in a minimum of _______ locations on each gear.
4
How are the mesh and backlash of spiral bevel gears adjusted?
by shimming the crown or pinion gear
How is backlash measured in worm and worm wheel reducers?
by restricting the rotation of one gear (usually the pinion) and measuring the rotational movement of the other gear at its pitch circle diameter
What are 3 common ways of checking backlash?
- dial indicator
- lead wire
- feeler gauges
What must be done to get an accurate measurement of backlash using the lead wire method?
the wire must be measured in two locations, as the wire has been squeezed between the mating teeth
What is a good practice when installing a new gearbox onto a sole plate?
to place 1/8” shims below the gearbox to allow for downward movement during alignment
What issues can soft foot cause in a gearbox?
distortion of the internal parts leading to binding, heating, and eventual lubrication and bearing failure
What devices must be used when a gearbox is mounted on the output shaft?
an anti-rotational device, such as a flange mount or torque arm
Where should shaft-mounted reducers be mounted to work best and last the longest?
as close as possible to a support bearing
What mounting precautions must be taken when using a tie rod (torque arm) as an anti-rotation device on a shaft mounted reducer?
it must be mounted in a position that forms a right angle between the tie rod and a line running from the output shaft to the point of attachment
When V belts are used to drive a shaft mounted reducer, how must the V belts be orientated?
at a 90º angle between output and centre of V-belt drive
This type of direct mechanical drive uses the motor shaft and the input shaft as one complete unit.
gear motor units
Why are most shaft-mounted reducers V-belt driven?
cost effectiveness
When dealing with a high-ratio planetary drive, what is reverse torque lockup?
a condition where the gears bind up in the reverse direction and stop any reverse motion
What precautions must be taken when working on planetary gear drives?
they can act as backstops, so the driven unit can supply energy back into the gearbox
What type of fasteners should be used when assembling a gearbox after overhaul?
New grade 8 fasteners with star washers or lock wire
What can happen to copper tubing in gearboxes?
they can work harden due to vibration and pressure pulses
Gearbox lubricant, at the minimum, should be changed every 1.____________ service hours or after 2.______ years.
- 20,000 hours
- 4 years
Bearings in gearboxes should be cleaned and packed with fresh grease every __________ service hours.
10,000 hours
- only fill to 1/3 of housing
What should be used to remove sharp-edged high spots and burrs on a gear tooth?
a tapered oilstone and light oil
- also called an abrasive file
What should be used to finish the surface of a tooth after it’s been cleaned up with an abrasive file?
2800 grit aluminum oxide stone
How is a residual magnetism check done on a gearbox?
with a quality compass
True or false:
The finish on the shaft at the seal contact area should be a 10 to 20 micron finish done by a plunge grinder.
true
What does the gear term backlash refer to?
the arc clearance between mounted, mating gears
Why would you number a gear in four location with numbers stamped on the side of the gear?
to allow backlash and mesh pattern checks over 360º or at four locations to establish gear and shaft integrity
Damaged gear teeth must be hand-worked to remove burrs and high spots. What tools can you use to dress damaged teeth?
abrasive files
What is used for checking for gear teeth cracks?
NDT dye penetrant and developer
Why is residual magnetism dangerous to rebuilt gearboxes?
the magnetism attracts iron particles to wear surfaces
Define P.I.V as it pertains to power transmission components.
positive infinitely variable
True or false:
The turnbuckle on a shaft mounted gearbox can be used as a belt tensioner.
true
How is the speed varied on a tube drive scoop?
By changing the level of fluid in the case
Which set of bevel gear sets cross over at 90° and have extensive sliding friction?
hypoid gears
How can the worm and worm wheel be designed so they have the maximum amount of tooth contact?
double-throated
If straight bevel gears are used at angles other than 90°, what are they called?
angular bevel gears
What formula can be used to calculate pitch circle diameter of a gear if the tooth count and diametral pitch of the gear are known?
PCD = # teeth ÷ DP
PCD = pitch circle diameter
# teeth = number of teeth on the gear
DP = diametral pitch of the gear
What would the final output be on a planetary gear set if none of the planetary members are held while there still being an input?
a neutral drive
What would the final output be on a planetary gear set if two of the gear members were locked together?
a direct drive output
How can the output and speed change be determined in a planetary gear set when the held and input members are known?
- 3 vertical dots labelled from top to bottom R, C, and S (ring, carrier, sun)
- ensure that the distance between C and S is double that of R to C
- mark the held member with an X
- draw a vertical line to the right from the input member
- draw a vector starting at the held member and intersecting the end of the horizontal line from the input
- draw a line from the output to the vector; this will determine speed increase (longer line than the input line) or decrease (shorter line that input line) as well as direction
- direction is based off of input line ⟶ if you draw the input line to the right, that is forward. if the resultant vector stays on the right, it is the same direction, if it ends up on the left, it is a reverse direction
AGMA suggests that backlash measurements indicate that as diametral pitch increases:
backlash increases
The larger the DP, the smaller the DP number
- average backlash = 0.04” ÷ DP
The worm gear set with the most amount of tooth contact pressure is:
non-throated
Hypoid gear sets utilize a __________ shaft orientation.
crossing over
The mechanical advantage of a multiple reduction gearbox is equal to:
The product of the gear ratios within the gearbox.
The more ____________ on the worm gear, the faster it advances the worm wheel.
starts
True or false:
Most often the end float of shafts within gearboxes must be near zero when measured on a dial indicator.
True
What are two advantages of a spring-mounted torque arm?
- Allows for some torsional flexibility (twisting)
- Absorbs shocks
What type of device is found in gearboxes and is used to remove excess oil off of gears and direct them into passageways that lead to bearings?
Oil scrapers
What 3 things cause destructive pitting?
- Contaminated lubricants
- Excessive heavy loads
- Low speed conditions (lack of lubricant)
This type of gear set crosses over and are at 90 degrees, but usually the two gears are not at the same centreline.
Hyoid gear sets
True or false:
Crowning aids with preventing seizing of gears.
True
With single helical gears, what bearings must be installed in the gearboxes?
Thrust bearings due to the axial torque
True or false:
Reversing gearboxes containing single helical gears requires thrust bearings on both ends of the shaft.
True
This type of bevel gear is stronger than others due to their cross over shaft orientation that allows the pinion gear to be straddle mounted.
Hypoid gear
- Which is the strongest type of bevel gear?
- Why is this?
- The Hypoid gear
- Due to the pinion being larger because of their ability to be straddle mounted
Which friction action is better for extending the life of a gear: rolling or sliding?
Rolling
- less friction
What can effect the rotation of a gear train if the number of gears are known?
If all of the gears mesh, or if some of the gears are on the same shaft
- gears on the same shaft count as one gear, so the total number of meshed gears would be reduced by 1 per gears that share the same shaft
What is the most common single-throated worm gear set arrangement?
throated worm wheel and non-throated worm
What does the contact pattern of meshing gears indicate?
the relative gear position
What types of loads do Zerol gears not develop: radial or axial?
axial;
- they do not develop axial loads
Identify the following gear geometries:
A = straight bevel
B = Zerol bevel
C = spiral bevel
D = hypoid
What is the shaft arrangement of an opposite hand helical gear set?
parallel shafts
What is the shaft arrangement of a hypoid gear set?
crossing over
What is the shaft arrangement of a mitre gear set?
intersecting
What is the shaft arrangement of a herringbone gear set?
parallel
What is the shaft arrangement of a worm gear?
crossing over
What is the shaft arrangement of a spiral bevel gear?
intersecting
What is the shaft arrangement of a Zerol bevel gear set?
intersecting
What is the shaft arrangement of a same hand helical gear set?
crossing over
What is the shaft arrangement of spur gears?
parallel
What is the shaft arrangement of double helical gears?
parallel
True or false:
When using Prussian blue to check for gear mesh on a worm and worm wheel gearbox, it is preferable to put the Prussian blue on the worm wheel rather than the worm.
True
What are 3 common ways to mount gearboxes?
- Foot mounted (most common)
- Flange mounted
- Shaft mounted
What determines the direction of thrust of a helical gear?
the direction of rotation
- while the helix angle also determines this, the main component will be the direction of rotation
