Mechanical power transmission system takes help of several drives and elements in order to transmit power, torque and motion from one shaft to another shaft. Such drives can be either engagement type (gear drive and chain drive) or friction type (belt drive and rope drive). Belt drive is one reliable mechanical power transmission system that is commonly used to transmit power and motion over considerably larger distance (even up to 15m) between driving shaft (usually a prime mover such as an electric motor) to driven shaft. Being a friction drive, belt drive is inherently associated with slip. So it cannot provide constant velocity ratio, thus it is one non-positive drive. This slip, however, can protect the driver unit from overloading in the driven part. Presence of flexible belt between two shafts also enables this drive to absorb vibrations induced in machinery. This belt many have various configurations, such as flat belt, V-belt, ribbed belt, etc. A flat belt is one jointed belt that has a rectangular cross-section. Only one surface of the belt comes in contact with the outer surface of the cylindrical pulleys.
Pulleys of the driver and driven shafts can be connected using the flat belt in two distinct ways—open belt and crossed belt. Each of them has unique benefits over the other one. In open belt drive arrangement, belt proceeds from top of one pulley to the top of other pulley without crossing. So the driver shaft and driven shaft rotate in same direction. Contrary to this, in crossed belt drive, belt proceeds from the top of one pulley to the bottom of other pulley and thus crosses itself in between two pulleys. Here driving shaft and driven shaft rotate in opposite directions. It offers higher contact angle, so power or torque transmission capacity also increases. However, due to crossing, belt continuously rubs itself, which leads to reduced belt life. Various similarities and differences between open belt drive and crossed belt drive are given below in table format.
Similarities between open belt drive and crossed belt drive
- In both the cases, flat belt is used. V-belt and ribbed belt cannot be utilized in crossed configuration (they are always open).
- In both the cases, driving and driven shafts must be parallel. However, a small misalignment does not possess noticeable problem.
- Both can transmit power and motion for substantially large distances (even up to 15m). In comparison, trapezoidal belt or V-belt is suitable for short distance (usually within 1m).
- In both the cases power transmission occurs by means of friction between the pulley and belt.
- Slip can occur in both the cases. So none of them can offer constant velocity ratio. Thus they are non-positive drive.
Differences between open belt drive and crossed belt drive
| Open Belt Drive | Crossed Belt Drive |
|---|---|
| In open belt drive, belt proceeds from top of one pulley to the top of other pulley without crossing. | In crossed belt drive, belt proceeds from top of one pulley to the bottom of other pulley and thus crosses itself. |
| In open belt drive, driving shaft and driven shaft rotate in same direction. | In crossed belt drive, driving shaft and driven shaft rotate in opposite direction. |
| Contact angle (or wrap angle) between the belt and pulley is comparatively small (always below 180º in smaller pulley). | Contact angle between the belt and pulley is comparatively large (always above 180º in smaller pulley). |
| Length of the open belt is smaller as compared to cross belt. | For the same pulley diameter and same centre distance between driver and driven shafts, longer belt is required in cross belt drive. |
| Here belt remains in same plane in every rotation during its operation. | Here belt bends in two different planes in every rotation during its operation. |
| Here belt does not rub with itself. So belt life is considerably high. | Here belt rubs with itself and thus life of the belt reduces. |
| Open belt drive is suitable when driving and driven shafts are in horizontal or little bit inclined. | Cross belt drive can be advantageously applied for horizontal, inclined and vertical positions of driving and driven shafts. |
| Power transmission capacity is small due to smaller wrap angle. | It can transmit more power as wrap angle is more. |
References
- Introduction to Machine Design by V. B. Bhandari (McGraw Hill Education India Private Limited).
- A Textbook of Machine Design by R. S. Khurmi and J. K. Gupta (S. Chand).
- Theory of Machines by R. S. Khurmi and J. K. Gupta (S. Chand).