PE Mechanical Machine Design flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Worst-case tolerance stack-up

Total assembly variation equals the arithmetic sum of all individual component tolerances, assuming every dimension is simultaneously at its extreme limit; this gives the largest possible variation and a conservative design.

Root-sum-square tolerance stack

Statistical stack-up that takes the square root of the sum of squared individual tolerances; valid when dimensions vary independently and randomly, producing a narrower and more realistic assembly tolerance than worst case.

Clearance fit

A fit in which the largest permissible shaft is smaller than the smallest permissible hole, so a gap always exists between the mating parts; used for sliding, running, or lubricated joints.

Interference fit

A fit in which the smallest shaft is larger than the largest hole, so assembly needs force, heating, or cooling; the resulting contact pressure transmits torque and prevents relative motion.

Bonus tolerance at MMC

When a feature departs from maximum material condition, the permitted geometric tolerance increases by the amount of that departure; a hole at its largest size receives the largest position tolerance.

Factor of safety

Ratio of a material strength, such as yield, ultimate, or endurance strength, to the applied stress; a value greater than one indicates margin against the selected failure mode.

Present worth factor

Converts a future single payment to today's equivalent value using P = F/(1+i)^n, where i is the interest rate per period and n is the number of periods.

Coefficient of variation

Standard deviation divided by the mean, expressing scatter as a dimensionless fraction of the average; it allows comparison of variability between data sets with different units or magnitudes.

Basic hole system

A limit system in which the hole lower deviation is zero, so the minimum hole equals basic size and all clearance or interference variation is obtained by shifting the shaft limits.

Normal distribution empirical rule

For a normal distribution roughly 68 percent of values lie within one standard deviation of the mean, 95 percent within two standard deviations, and 99.7 percent within three.

Renard preferred number series

Standard progressions R5, R10, R20, and R40 with step ratios of about 1.6, 1.25, 1.12, and 1.06, used to standardize sizes and reduce product variety.

Goodman fatigue criterion

Fatigue criterion joining endurance limit on the alternating-stress axis to ultimate strength on the mean-stress axis; failure is predicted when sigma_a/Se + sigma_m/Sut reaches 1/n.

Soderberg fatigue criterion

Conservative fatigue criterion that uses yield strength in place of ultimate strength: sigma_a/Se + sigma_m/Sy = 1/n; its line always lies inside the Goodman line for ductile metals.

Gerber fatigue criterion

Parabolic fatigue criterion connecting endurance limit to ultimate strength: sigma_a/Se + (sigma_m/Sut)^2 = 1/n; it is less conservative than Goodman and fits ductile steel data well.

Marin modifying factors

Part endurance limit equals the rotating-beam specimen limit multiplied by surface, size, load, temperature, reliability, and miscellaneous factors: Se = ka*kb*kc*kd*ke*kf*Se'.

Steel endurance limit estimate

For wrought steels with ultimate strength below about 1400 MPa (200 ksi), the fully reversed rotating-beam endurance limit is approximately half the ultimate tensile strength; above that it is about 700 MPa.

Fatigue notch factor Kf

Defined as Kf = 1 + q(Kt - 1), where Kt is the theoretical stress concentration factor and q is notch sensitivity, ranging from zero for insensitive ductile metals to one for sharp notches.

Bearing rating life L10

Life in millions of revolutions that 90 percent of a group of identical bearings survive: L10 = (C/P)^a, with a = 3 for ball bearings and a = 10/3 for roller bearings.

Lewis gear bending equation

Gear tooth bending stress sigma = Wt/(F*m*Y), where Wt is the tangential load, F the face width, m the module, and Y the Lewis form factor based on tooth count.

Joint stiffness constant C

The fraction of external tensile load carried by a bolt: C = kb/(kb + km), where kb and km are the bolt and clamped-member stiffnesses; typical values fall between 0.2 and 0.3.

Euler buckling load

Critical load for a long slender column: Pcr = pi^2*E*I/(K*L)^2, where the effective length factor K is 0.5 fixed-fixed, 1.0 pinned-pinned, or 2.0 fixed-free.

Wahl factor

Curvature-corrected stress factor for helical springs, Kw = (4C-1)/(4C-4) + 0.615/C, where C = D/d is the spring index; it accounts for direct shear and coil curvature.

Fillet weld throat

The effective throat of a fillet weld equals 0.707 times the leg size; weld area is throat times length, and shear stress is the applied force divided by that area.

Jominy end-quench test

Hardenability test in which a standardized bar is water-quenched at one end and hardness is measured along its length; hardness versus distance shows how deeply the steel will harden.

Tempering of quenched steel

Reheating quenched steel below the transformation temperature relieves internal stress and lowers hardness while raising toughness and ductility; higher tempering temperatures produce lower strength.

Carburizing

Case-hardening process that diffuses carbon into the surface of low-carbon steel at elevated temperature, creating a hard, wear-resistant case over a tough, ductile core.

Gray cast iron

Cast iron with flake graphite that provides excellent vibration damping, machinability, and compressive strength but very low tensile ductility, since the graphite flakes act as internal stress raisers.

Aluminum 6061-T6

Heat-treatable aluminum-magnesium-silicon alloy in the solution-treated and artificially aged condition, with tensile strength near 310 MPa and yield near 275 MPa; it is weldable and corrosion resistant.

Carbon equivalent

CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15; values above about 0.4 indicate poor weldability and require preheat to avoid cracking in the heat-affected zone.

Shot peening

Cold working the surface with small spherical media induces compressive residual stress, which raises fatigue strength and resistance to stress-corrosion cracking by inhibiting crack initiation.

Annealing

Heating steel above its critical temperature and then cooling slowly in the furnace produces soft, stress-free, coarse pearlite with maximum ductility and improved machinability.

Galvanic corrosion

Accelerated attack occurring when two dissimilar metals contact in an electrolyte; the more anodic metal corrodes faster, and severity grows with the difference in galvanic series potential.