Stiff shaft design

Stiff shaft designs have eliminated shaft and bearing eritieals in the entire operating range. 

The stiff shaft design has lower clearance requirements, which allows non-contacting seals to be used. Contacting seals, if used, would... 

Fig ure 5-18. Finish-machined shaft showing integral thrust collar. The rotor is a stiff-shaft design. 

The shaft is a forging and may he designed as a stiff shaft or flexible shaft. A stiff shaft design means that the shaft will operate helow any of its critical speeds. Usual practice limits design operation to 60% of the first critical speed. This requires a heavier shaft than the flexible design that allows the shaft to pass through its first critical speed at 40-60% of normal and maximum operating speeds. 

The design of shaft seals must strive to reduee wear of these eritieal eomponents. An overhung turbine design should use a stiff shaft that operates below its first shaft eritieal speed. This will greatly limit the extent of damage in the event of aeeidental or... 

The expander design will be of axial entry (Figure 5-14), overhung rotor, stiff shaft construction with one or two rows of blading designed and constructed for 100,000 hrs of life and at least 3 yrs of uninterrupted continuous service. It is recognized that uninterrupted operation for this period of time involves factors beyond the Seller s control. 

Using a rear-wheel-drive vehicle as an example, a prop shaft delivers power from the transmission output, near the middle of the vehicle, to the differential that drives the wheels at the rear. If this prop shaft is designed to be stiff, which is normally the case, it could not be rigidly attached to the transmission output at one end and the differential at the other because of the differences in vertical movement between the drive wheels and the chassis. Typically, two universal joints are inserted into the drivetrain to accommodate this situation. 

Hence the dilemma faced by a designer of a deep rope guided shaft can be appreciated from this Table 3. The designer must either seek guide rope factor of safety dispensation from the statutory authorities or he must find other ways of providing extra lateral stiffness (more and/or heavier rope guides) or provide greater lateral clearances for the deep shaft. 

Shaft steel in the modern day is often designed using SANS 10208-4 as the basis of design. This approach is the most modern and deterministic currently available, and provides a good basis for relative stiffness and applied load considerations. 

The other key design assumption is that the support stiffness is sufficiently large that the overall stiffness, k, is controlled only by the shaft stiffness. With a stiff support the natural frequency depends only on the shaft stiffness and associated mass. Mixer manufacturers generally assume that the mixer will be mounted on a strncture where a small change in stiffness does not significantly affect the natnral frequency. 

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