Gearing and Variable Speed Drives Flashcards
What are gears?
Positive power transmission devices that do not slip
What are the two types of gear drives?
Open gear drives and enclosed gear drives
Which gear drive system has more wear and why
The open drive system has more wear because of contaminants
How are you enclosed gear drives lubricated?
By either grease or oil
What is a gear set
Matched pairs of gears manufactured together
What is a gear train?
When two or more gears mesh or work together
What is a pinion?
When two gears have a different number of teeth, the lower number is called the pinion
What is a crown gear?
In a bevel and hypoid gear set, the larger set is called a crown gear or a ring gear
What is drive and driven gear
Drive=power in
Driven=power out
What is direct drive?
When the input and output is the same speed. It is often used to change directions
What happens when speed is reduced?
The output has an increase in torque. The result is a mechanical advantage
What is an overhung gear?
It has a bearing support on only one side
What is a straddle mounted gear?
Gears that are mounted between bearings
What is a compound gear train?
They have three or more shafts and four or more gears
What is preloading of bearings?
Giving the bearing less than zero axial clearance. This is common with tapered roller bearings in a worm gearbox
What can an increase in backlash be an indication of?
Gear wear
What is a floating gear or shaft?
Where the gear or shaft is allowed to float axially to its best position
Describe how to tell if a helix angle gear is LH or RH
When viewing from the shaft angle, if the teeth point up to the right, it’s a RH and vice versa
Parallel shafts must have what combination of angled gears?
Must use opposite hand gears
Crossing over shafts must have what combination of angled gears?
They must have the same hand gears (such as in a worm gear)
Describe a worm and worm wheel gear set
The worm is the smaller gear, and the worm wheel is is the larger circle gear
What is the drive side of gear teeth?
Most gearboxes rotate in one direction most or all of the time, so the wear is I the drive side of the teeth, and the side that doesn’t make contact is the coast side
What is the addendum?
The top half of the teeth
What is the dedendum?
The bottom half of the tooth
What is the whole depth?
The addendum plus dedendum
What is the clearance?
The space between the bottom land of one gear, and top land of the meshing gear. It allows for lubricant, thermal expansion, and gear runout
What is the working depth?
The addendum, plus the dedendum, minus the clearance
What is the circular pitch?
Also called the gear pitch, it is the distance from the center of one tooth to the center of the next tooth, at the pitch circle
What is the pitch circle?
The diameter, at the pitch circle. It is approximately in the middle of the gear teeth
What is crowning?
A slight curve on the face of the tooth from side to side
What is pressure angle?
It is the angle on the side of the tooth. Also known as the angle on the line of action which is the point of contact as the gear turns. The two common angles are 20° and 14.5°
What is a helix angle?
The angle of the tooth from the shaft axis of the gear is called the helix angle
Why are steeper helix angles stronger and smoother tooth action?
They are stronger because they have a longer line of contact however they have more sliding friction and create axial thrust
What is a start?
A term that describes the number of threads starts on the worm in a worm gear box
What is backlash?
It is the clearance between the gear teeth at the pitch line
What is diametral pitch?
It indicates to the size. It is the number of teeth on the gear divided by the pitch diameter
Binding can because by several factors. What are they?
Lack of backlash or clearance between the meeting Teeth, incorrect center distance between gears, gear center out of tolerance, bent shafts or shaft deflection
For gears to mesh properly, what must happen?
Same diametral pitch
Same pressure angle of the teeth
Correct hand of gear
Shafts in correct position
Same helix angle
How do you determine the rotation of odd and even gear combinations?
Even number of gears=opposite direction
Odd number=same direction
What do idler gears do?
Main function is to change direction. They can also change the center distance
What are the three common shaft positions?
Parallel, intersecting, crossing over at 90
What are spur gears?
Straight teeth, must be used in parallel shaft position. They generate no axial thrust.
What does a rack and pinion gear set do?
Transfers circle motion into linear motion
When is there maximum power transmission between teeth?
At the pitch line when the teeth meet it becomes a rolling action, that is the maximum transfer point. As they are coming into mesh it is a sliding action
What do some helical gears need in the oil?
They need an EP additive in the oil
What is a gear ratio of a mitre gear set?
They are always 1:1 and always at 90 degrees
Why are helical gears popular?
They are stronger because of the linear contact and have more teeth in contact at one time
What is benefit of double helix gears?
The opposite hand helixes cancel out the axial thrust
What are herringbone gears?
They are similar to double helical gears, but there is no gap in the middle, it comes to a point. They are recommended for high speed and heavy loads.
What are spiral bevel gears?
Stronger than straight bevel gears, and give a smoother transfer of power because they have more teeth in contact at the same time.
What is different about zerol bevel gears?
Because the teeth are curved they have more contact area, but the produce no axial thrust
Why are hypoid gears stronger than spiral bevel gears?
They have a larger pinion, have extensive sliding action, and because they are crossing over shafts, it can be straddle mount
What makes a worm gear self locking?
1 or 2 starts, and usually 30:1 ratio at least
What is worm lead?
The distance by which one tooth on the worm will advance in one turn
PitchX# of starts=lead
How do you determine gear ratio of a multistart worm gear?
worm wheel teeth divided by number of starts
What is a throated worm?
It is a worm shaped like an hourglass
Which arrangement gives the most tooth contact in a worm gear drive.
A double throated gear set
How should the worm wheel be set up?
It should be set up so the contact is slightly towards the leaving side of the worm wheel
What are the components of a planetary gear set?
Sun gear, planetary gears with carrier, ring gear
What are concentric reducers?
The input and output shafts are on the same plane
What is an offset horizontal drive gearbox?
The input and output shafts are on a different plane.
How must torque arms be aligned?
90° angle between output and point of torque arm
What device do gearbox manufacturers use to absorb torsion shock load?
Spring type torque arm
Why is residual magnetism in rebuilt gearboxes bad?
Magnetism attracts and particles to wear surfaces
A good mesh pattern between gears that are mated is indicated by?
A central tooth pattern and not contacting on either edge
What is a mating gear mesh pattern?
The shiny metallic surfaces that show up when one tooth drives another
What are the two operational ranges that all variable speed mechanical drives operate in?
Constant torque and constant horsepower
Constant torque is when output speed is below input speed
How fast can leaf chain drives go?
Below 3000 rpm
Describe a PIV
A positively infinite variable speed reducer. Only good up to 900 rpm and requires EP lubrication
When should oil be changed in a chain drive?
Every 2000 hours. The oil will be full of metal flakes. This is normal
What are the three major components of the torque converter?
The input impeller, the follower turbine, and the stater
How is output speed and power changed on the variable Fill scoop tube hydraulic Drive?
By changing the amount of oil in the rotating case
What are the three basic spur gear arrangements?
External, internal, rack and Pinion
