Mechanical Stiffness

When output motion must respond to a rapidly changing input
command, the control system must have a wide bandwidth. Where the load mass (in linear motion systems) or the polar moment of inertia (in rotary systems) is high, there is a possibility of resonant oscillations. For the most stable and reliable systems, with a defined load, a high system mechanical stiffness is preferred. To attain this stiffness requires strengthening shafts, preloading bearings, and minimizing free play or backlash. In the best-performing systems, motor and load are coupled without intervening compliant members. Even tightly bolted couplings can introduce compliant oscillations resulting from extremely minute slippages caused by the load motions.
Backlash is a factor in the effective compliance of any coupling but has little effect on the resonant frequency because little energy is exchanged as the load is moved through the backlash region. However, even in the absence of significant torsional resonance, a highgain control system can “buzz” in the backlash region. Friction is often sufficient to eliminate this small-amplitude, high-frequency component. The difficulty with direct-drive control systems lies in matching motor to load. Most electric motors deliver rated power at higher speeds than are required by the driven load, so that load power must be delivered by the direct-drive motor operating at a slow and relatively inefficient speed. Shaft power at low speed involves a correspondingly high torque, which requires a large motor and a high-power controller. Motor copper loss (heating) is high in delivering the high motor torque. However, direct-drive motors provide maximum
load velocity and acceleration, and can position massive loads within seconds of arc (rotational) or tenths of thousandths of an inch (linear) under dynamic conditions.

Where performance requirements are moderate, the required load torque can be traded
off against speed by using a speed-changing transmission, typically, a gear train. The transmission effectively matches the best operating region of the motor to the required operating region of the load, and both motor and controller can be much smaller than would be needed for a comparable direct drive.

Chromium Nickel Austenitic Steels

Chromium Nickel Austenitic Steels (Not capable of heat treatment).—SAE 30201:
This steel is an austenitic chromium–nickel–manganese stainless steel usually required
in flat products. In the annealed condition, it exhibits higher strength values than the corresponding
chromium–nickel stainless steel (SAE 30301). It is nonmagnetic in the annealed
condition, but may be magnetic when cold-worked. SAE 30201 is used to obtain high
strength by work-hardening and is well suited for corrosion-resistant structural members
requiring high strength with low weight. It has excellent resistance to a wide variety of corrosive
media, showing behavior comparable to stainless grade SAE 30301. It has high ductility
and excellent forming properties. Owing to this steel’s work-hardening rate and yield
strength, tools for forming must be designed to allow for a higher springback or recovery
rate. It is used for automotive trim, automotive wheel covers, railroad passenger car bodies
and structural members, and truck trailer bodies.
SAE 30202: Like its corresponding chromium–nickel stainless steel SAE 30302, this is a
general-purpose stainless steel. It has excellent corrosion resistance and deep drawing
qualities. It is nonhardenable by thermal treatments, but may be cold worked to high tensile
strengths. In the annealed condition, it is nonmagnetic but slightly magnetic when coldworked.
Applications for this stainless steel are hub cap, railcar and truck trailer bodies,
and spring wire.
SAE 30301: Capable of attaining high tensile strength and ductility by moderate or
severe cold working. It is used largely in the cold-rolled or cold-drawn condition in the
form of sheet, strip, and wire. Its corrosion resistance is good but not equal to SAE 30302.
SAE 30302: The most widely used of the general-purpose austenitic chromium–nickel
stainless steels. It is used for deep drawing largely in the annealed condition. It can be
worked to high tensile strengths but with slightly lower ductility than SAE 30301.
SAE 30303F: A free-machining steel recommended for the manufacture of parts produced
on automatic screw machines. Caution must be used in forging this steel.
SAE 30304: Similar to SAE 30302 but somewhat superior in corrosion resistance and
having superior welding properties for certain types of equipment.
SAE 30305: Similar to SAE 30304 but capable of lower hardness. Has greater ductility
with slower work-hardening tendency.
SAE 30309: A steel with high heat-resisting qualities which is resistant to oxidation at
temperatures up to about 1800 degrees F.
SAE 30310: This teel has the highest heat-resisting properties of any of the chromium
nickel steels listed here and will resist oxidation at temperatures up to about 1900 degrees
F.
SAE 30316: Recommended for use in parts where unusual resistance to chemical or salt
water corrosion is necessary. It has superior creep strength at elevated temperatures.
NUMBERING SYSTEMS 429
SAE 30317: Similar to SAE 30316 but has the highest corrosion resistance of all these
alloys in many environments.
SAE 30321: Recommended for use in the manufacture of welded structures where heat
treatment after welding is not feasible. It is also recommended for use where temperatures
up to 1600 degrees F are encountered in service.
SAE 30325: Used for such parts as heat control shafts.
SAE 30347: This steel is similar to SAE 30321. This niobium alloy is sometimes preferred
to titanium because niobium is less likely to be lost in welding operations.