Belt Drives

Owned by mcolabre@uwaterloo.ca
May 17, 2021
3 min read

Types of Belts

Flat Belts

  • Simple rectangular cross-section

  • One or both sides can be specially coated to increase friction with pulleys

  • Pulley is slightly convex to keep the belt centered on the pulley and prevent it from falling off

  • ~98% efficiency

  • High flexibility (can be used effectively with small pulleys)

  • High pretensioning forces/bearing load

  • Use outer diameter of the pulleys to calculate transmission ratio
 Pictures

Belt (mechanical) - Wikipedia

V-Belts

  • Reduced bearing load

  • Wedge-shaped cross section

  • Higher friction than flat belts due to wedge shape
  • Higher torques can be transmitted with the same pretensioning force
  • Two+ belts can be placed parallel to each other to increase power transmission as well
  • Belt only contacts the pulley on the inclined flanks (does not touch the bottom of the groove)
  • Groove angle of 38° or 32° depending on pulley diameter
  • Each belt is specially designed for a certain range of pulley diameters
  • Different way of calculating transmission ratio (nominal width/neutral axis)
 Pictures

Standard V-belt (wrapped)

Type of V-BeltDescription
Classical
  • Height to width ratio of 1:1.6
Narrow
  • Height to width ratio of 1:1.2
  • Smaller width with the same power transmission
    • Or higher power transmission with the same width
  • Higher speeds (because of lower mass)
  • Less flexible
    • Can come in cogged form (notches, that kind of look like teeth, cut out of the bottom) to increase flexibility
  • Higher efficiency than classical
Wide (a.k.a. variable speed)
  • Height-width ration of 1:2+
  • Good for heavy power transmission and when large speed changes occur
  • Usually cogged
Double (a.k.a hex)
  • Two V-belts back to back, basically
  • Can drive pulleys on either side
Kraftband
  • Multiple V-belts connected by a cover plate
  • Helps ensure individual belts don't fall off the pulley
Coating TypeDescription
Wrapped
  • Covered with a special rubber fabric
  • Increases friction
  • Protects belt form harmful environments
Raw
  • No rubber casing
  • More flexible
  • Higher power transmission and efficiency

  • Can be ground to narrower tolerances

Timing/Synchronous Belts

  • Toothed to eliminate slippage
  • Used when precise positioning is required
  • Higher power density (can use smaller belts and pulleys for a bigger load)
  • More expensive
  • Require less tension (and cause less wear)

Round Belts

  • Round cross section
  • Used more for motion transmission than power transmission

Chains

  • Longer length
  • Better in high temperatures
  • Needs lubrication and more maintenance

Timing Belt Specifics

The list above is an introduction I guess, but really we want to be using timing belts. Any other type of belt doesn't have teeth and relies only on friction, causing it to slip occasionally, which is bad for our purposes.

Tooth Profiles

Note: Belts with the same type of tooth profile and pitch may not be interchangeable, due to differences in manufacturer's designs

TypeDescriptionProsConsPicture
Trapezoidal
  • Trapezoidal shaped teeth
  • Most common
  • Can transport good amounts of force
  • Low backlash
  • High wear rates (with high torque/speeds)

tooth profiles

Curvilinear
  • Rounder teeth
  • Improved torque and speed capabilities
  • Less belt ratcheting (jumping)
  • Higher backlash than trapezoidal

tooth profiles

Modified Curvilinear
  • Slight improvements to the curvilinear design
  • Highest torque and force transmitting capabilities
  • Least likely to ratchet
  • Higher backlash than trapezoidal

tooth profiles

Pulley Mounting

TypeDetails
Set screws
  • Low and medium loads
  • Low cost
  • Pulley location can't be adjusted easily if shaft is keyed
Split hub
  • Shaft collar with a clamping screw
  • Low cost
  • Medium loads
  • Moren easily adjustable than set screw + keyed shaft
Quick disconnect bushing
  • Slide on and attach to face of pulley
  • Tighten on the shaft as they are tightened
  • High loads

Taper lock

(a.k.a keyless bushing)

  • Similar mounting to QD bushing but inside the pulley(?)
  • High loads
  • Flexible for design
  • Can only mount on one side

Theory

https://www.thomasnet.com/pdf.php?prid=101106

ConceptSymbolDefinition
Belt pitchpThe distance between the centerlines of two adjacent teeth
Belt pitch line

The centerline of the cords inside the belt (near the top)

Pulley pitch
The arc length between the centerlines of two adjacent pulley grooves
Pitch circle
The pitch line of the belt when wrapped around the pulley. In timing belts, the pitch diameter is larger than the pulley outside diameter.
Pitch diameterdThe diameter of the pitch circle
Pulley outside diameter
The diameter measured from the ends of the teeth
Belt lengthLIf zb = number of teeth, L = p⋅zb
ConceptSymbolCalculationDefinition
Centre distanceC

The distance between the centers of the two pulleys.