Spindle
Accessories
A basic idea about belt-driven spindle design
A belt-driven spindle design consists of the spindle shaft, held with a bearing system and supported by the spindle housing. The entire tooling system, including the tool taper, draw bar mechanism and tool release system is incorporated in the spindle shaft. An external motor supplies power and rotation to this spindle. The motor is mounted adjacent to the spindle. The torque is transferred to the spindle shaft by using a cogged or V-belt. The power, torque and speed of the spindle hence depend upon the specifications of the driving motor, and the belt ratio used between the motor and the spindle.
Advantages
The main advantages of the belt-driven spindle design are as follows:
- Reasonable cost: The spindle is made up of very few basic
parts and hence the cost is relatively low.
- Wide variety of spindle characteristics: Another main
advantage is that the final specifications or features can be modified
for a particular application by using a different motor or belt ratio.
This is because the spindle power, speed and torque are dependent upon
the driving motor. In some cases, gears are also used in addition to the
fixed belt ratio to provide multiple speed ranges.
- High power and torque possible: Since the spindle motor is fixed outside the actual spindle shaft, it is often possible to use a very large motor. A large motor can provide very high torque and high power for spindle use. This is not possible in an integral motor-spindle design because available space is always limited.
Disadvantages
However, there are also some disadvantages of a belt-driven spindle design which are as follows:
- Limited maximum speed: A belt-driven spindle has limited
maximum rotational speed due to many factors. The mechanical connection
which transfers the torque to the spindle shaft, the pulley and the belt
system, are limited in maximum operating speed. This depends on the type
of belt used. For example, if a poly V-belt system is used, high
rotational speeds tend to stretch and disengage the belts which in turn
reduces the ability to transmit torque. Cogged belts at higher speeds
produce unacceptable levels of vibration. Gears also produce high levels
of vibration and heat and are very limited in maximum speed.
- Belts Utilize Bearing Load Capacity: In order to transfer the necessary torque, belt-driven spindles use a belt and pulley connection on the end of the spindle shaft. The required tensioning of these belts leads to a constant force on the rear spindle shaft bearing set. The applied tension and consequent force increases as the power and speed of the spindle increase. This utilizes much of the available radial loading capacity of the bearings. And, adding or substituting additional bearing sets will not be feasible, because they will further reduce the spindle abilities to reach high rotational speeds.
Thus we come to the conclusion that a belt-driven spindle will be limited to certain applications. Typically, belt-driven spindles can operate up to maximum rotational speed of 12,000 - 15, 000 RPM. This also depends on other factors like bearings types, setups, or bearing lubrication.
A basic idea about integral motor-spindle design
While various types of spindles are being developed, integral motor spindle design remains the workhorse for high-speed machining. This design does not depend upon an external motor to provide power and torque. The motor is an integral part inside the spindle shaft and housing assembly, which makes easier for the spindles to rotate at higher speeds as a complete unit, without the additional limitations of gears or belts.
Typically, a complete motor-spindle comprises the spindle shaft, including motor element and tooling system. The spindle shaft is held by a set of high precision bearings, which require lubrication like grease or oil. The spindle shaft rotates up to the maximum speed, and display the power characteristics of the motor type that is used. The selection of a particular component depends upon the requirements of the machine tool.
Advantage
One of the distinct advantage of the integral-motor design is allowing the spindle to achieve the high speeds required for high-speed machining or high-stock removal applications, and also used in general applications using small diameter tooling. Spindle speeds more than 30,000 rpm are routinely achieved with spindles of integral motor design, while speeds in this range are considered highly impractical or problematic with belt-driven or gear-driven designs.
Disadvantage
One of the disadvantage that face the integral-motor spindle design is to provide wider speed ranges for improved applicability and to deliver more torque to achieve higher volume material removal rates.
