First pulley is considered to be a driver pulley. The rest of the pulleys are driven pulleys or idlers. Input power can be split among several driven pulleys by using power ratio factor of each pulley. The forces and torques are calculated accordingly. Flat pulleys are considered as idlers.
Service factor c P
Total service factor takes into account the safety factors required to compensate for belt life-reducing factors encountered during service, such as load, acceleration and fatigue. Load factor depends on the type of the driver and driven machine. The acceleration add-on factor c pa can be considered if speed up ratio is> 1.24, please see table below. Fatigue add-on factor takes into account operational hours per day and unusual service conditions.
|
Speed up ratio 1/i |
c PA |
|
1.00 - 1.24 |
0.0 |
|
1.25 - 1.74 |
0.1 |
|
1.75 - 2.49 |
0.2 |
|
2.50 - 3.49 |
0.3 |
|
3.5 and more |
0.4 |
Teeth in mesh factor k z
Teeth in mesh factor take into account the number of teeth in contact zc of the synchronous pulley. If the teeth in contact of the given synchronous pulley is less than 6 it can have significant impact on belt power capacity. Application finds a minimum value of teeth in contact among all synchronous pulleys within belt drive and then use following rule to obtain k z factor.
|
z c ≤ 6 |
k z = 1 |
|
z c < 6 |
|
Number of teeth in contact is determined based on arc of contact angle of each individual pulley as follows
Tension factor k 1
Tension factor gives an option to adjust belt initial tension. When belt drive operates under load tight and slack side develops. The initial tension prevents the slack side from sagging and ensures proper tooth meshing. In most cases, synchronous belts perform best when magnitude of the slack side tension is 10% to 30% of the magnitude of effective pull {k 1 = 1.1 ~ 1.3}.
Efficiency η
When properly designed and applied, belt drive efficiency is usually high as 96%-98% {η 0.96 ~ 0.98}. This high efficiency is primarily due to the positive, no slip characteristic of synchronous belts. Since the belt has a thin profile, it flexes easily, thus resulting in low hysteresis losses as evidenced by low heat buildup in the belt.
Belt length correction factor c L
Belt length correction factor takes into account modification of belt power rating of extreme belt length. By default the value is 1.0 what does not affect the results.
Resultant service factor c PR
The resultant service factor is determined from equation below. The belt power rating for given transmission layout is compared with power to transmit. The resultant service factor gives fast answer of how much the belt drive is over designed.
|
c PR < c P |
Strength check fails |
|
c PR ≥ c P |
Strength check succeeds |
|
c PR > c P |
Consider to change transmission layout, use different belt or decrease belt width |
Meaning of used variables:
|
z c |
Number of teeth in contact of given pulley [-] |
|
z |
Number of teeth of given pulley/ Number of belt teeth [-] |
| β |
Arc of contact [deg] |
|
P |
power to transmit [W] |
|
P R |
Belt power rating for given transmission layout [W] |
|
c p |
Service factor [-] |