6.9 transitions Flashcards
What are gears used for?
Mechanical advantages or to change the direction of movement
Why are gears used in conjunction with shafts and bearings?
To transmit power and reduce or increase rotational speed
What is a spur gear?
They have straight teeth parallel to the shafts axis
What do spur gears connect?
Parallel shafts only
What is bad about spur gears?
They can be noisy
When two spur gears mesh together, what is the larger gear called?
What is the smaller gear called?
The wheel gear
The pinion
When are external spur gears used?
When a change of speed is required and the shafts are parallel to each other
When are internal spur gears used?
When a change of speed is required whilst maintaining an overall minimum diameter
What are helical cut spur gears?
They connect parallel shafts but have their teeth cut at an angle
What is an advantage of helical cut spur gears?
Smoother and carry heavier loads. Also quieter and smoothly
What load do single helical gears transmit?
Axial
What does single helical gears having more teeth in a mesh do?
Provide a larger contact area so transmit more power
What are double helical gears?
They are two identical gears joined on the same axis with opposite teeth
What force does double helical gears act in and what needs to be in the receiving end?
Axial
What should a double helical gear degree not be larger than?
20 degrees
In wheels with herringbone teeth, what happens to the axial forces?
They cancel each other out.
When are herringbone gears used?
For transmission of large forces.
What are bevel gears?
Used between intersecting shafts and can be designed for any angle of intersection
Why are bevel gears quieter, smoother and can take heavy loads better than a size equivalent spur gear?
They have curved teeth
What are hypoid gears a progression of?
Spiral bevel gears
How do hypoid gears differ to spiral bevel gears?
The shafts axis are not on the same plane so do not intersect
What do hypoid bears suffer from and how is this fixed?
Heavy sliding contact so require constant lubrication
Are hypoid bears quitter and smoother than spur gears?
Yes
What are worm gears?
Non intersection shafts not always at right angles
What do worm gears consist of?
A worm screw that drives a worm wheel (similar to a spar gears)
What do worm gears offer?
High reductions in simple steps.
Why are worm gears inefficient?
Large sliding movement of their teeth
What are worm gears good at?
Being driven in both directions
What is rack and pinion?
A gear system that is used to transmit motion between a rotating spur gear and linear toothed rack
What forces do rack and pinion gears transmit?
Rotational motion into linear motion
What are rack and pinion used for?
Leading edge flaps for extension and retraction
When is a mechanical advantage not gained?
If the drive and driven gear have the same number of teeth
What is the direction of the shaft and the rotation of the neighbouring gear?
Opposite
What is the gear ratio?
Number of teeth on A
Number of teeth on B
What are idler gears also known as?
Intermediate gears
What is one thing an idler gear is used forr?
To make the output gear move in the same direction as the input gear
What is another thing an idler gear can be used for
To link gears where there is a gap between them
Does the presence of an idler gear make a difference to the ratio of speed?
No
For even numbers of gears from the input gear, what is the rotation?
For odd numbers of gears?
Opposite the first gear
The same
How is the direction of rotation calculated?
By counting the number of gears because the driven gear and the gear
What must engines operate at for high efficiency?
High speeds
High speeds
An arrangement by which an input speed can be lowered to obtain a slower output speed with the same torque
What does a reduction gear assembly consist of?
A set of rotating gears connected to a shaft
What is the reduction gear assembly known as?
The reduction gear box
What are epicyclic and planetary gears used for?
Reduction gearing in aircraft engines
whatan does a epicyclic gear consist of?
Two mounted gears so that the centre of one gear revolves around the centre of the other (planets around the sun)
What is the meshing and pattern of a gear determined by?
How the teeth mate
What can happen if the gears are meshed too high?
The load is transmitted to the smallest portion of the tooth breaking the teeth
Where is the ideal placement of teeth?
The middle
What can happen if the teeth are too low?
They will not lubricate
What can the pattern and meshing be?
Adjusted for direction
What is gear backlash?
The gap between the gear teeth and the meshing point to allow for lubrication and prevent overheating
What can gear backlash cause?
Lost motion (deviation) making it difficult to achieve accurate positioning
What is an indication of worn gearing?
Excessive backlash
What is a belt?
Used to link or drive two or more rotating shafts
When are belts often used?
In parallel and offer high speed power transmission that is resistant to slipping and misalignment
How can belts be used?
Can be driven in one direction
Can be crossed so that they work in reverse
Why else can a belt be used?
To change the speed of rotation, either up or down
What are two types of belts?
Friction and positive drive
What are friction drive belts?
They rely on friction between belt and pulley to transmit power. They require tension
What are positive drive belts?
Rely on the engagement of the teeth on the belt with grooves on the pulley. There is no slip apart from ratcheting or tooth jumping
What are leaf chains?
A simple form of steel chain only consisting of a link plates and pins
What is good about leaf chains? Compared to roller chains
They are strong and have high tensile strength compared to roller chains
What do leaf chains not have and how does this affect their work?
Roller or bushes meaning they are only to be used for low speed applications and lubricated regularly
What is an advantage of chain drives?
The chain cannot slip on the sprocket because of the teeth
What is a disadvantage of chain drives?
They are heavy noisy and expensive
Where are chain and sprocket gears used?
On areas were slipping must not occur
On areas were slipping must not occur
Sheaves and is fitted with a clevis at each end
What are roller chains comprised of?
Link plates and pins
Rollers and bushes
What must the size of the chain link match?
The size and spacing of sprocket teeth
What is a sprocket?
A toothed wheel that fits onto a shaft
How is a sprocket stooped from rotating on the shaft?
By locking it with by a key way or grub screw
How many sprockets do chains connect?
Two
Why are sprockets called power transmission chains?
They can transmit motion and force
Why are v belts used?
Low cost and ease of installation
Low cost and ease of installation
Low cost and ease of installation
What is the biggest advantage in the V belt?
The wedging action in the pulley groove
What do v ribbed belts consist of?
What does the rib ensure?
Fist belts and V belts
The belt tracks properly making alignment less critical
What is a positive (Synchronous) belt?
The tooth profile is a trapezia
design
What properties does a synchronous belt have?
A flat belt with evenly spaced teeth perpendicular to the axis of the belt
What do timing belts rely on?
Engagement with belt teeth on the sprocket, not friction
Where are timing belts used?
Where exact synchronisation between driver and driven shaft is required
What is a V belt pulley?
They have a geometry if the grooves located around the pulley to gain traction on a V belt
What are V belt pulleys used for?
Transmit power between two parallel axels
What do synchronous belt pulleys mate with?
The same pitch as synchronous belts
What are the advantages of synchronous belts?
They’re quiet, are non slip and non lubricated operation. They have low maintenance requirementsv
What is a stepped or cone pulley drive?
A system of pulleys made up from
What does a step pulley consist of in terms of configuration?
2 to 4
When can the same belt tension be achieved?
When the sum of both working stages are identical
What is good about V ribbed belts?
They can run at high speeds
Why is pulley alignment critical to flat belts?
Because there is no chance of belt slip off
What is not advised with flat belts?
High outputs at high speeds
What are modern belts made out of?
Aluminium and nylon with rubberised plastic
What do timing belts not rely on?
Friction
