Failure modes of machine elements.
1. How Rolling bearings fail. Which statement is untrue?
Most roller bearings fail by surface fatigue.
Most roller bearings outlive the machines. Only 0.5% are replaced because of bearing failure.
Failure analysis are important to be able to take corrective action to improve machine reliability.
It is evident that costs of an unexpected machine standstill in a production line can be extremely high.

2. How gears fail. Which statement is untrue?
Gear failures can be subdivided into tooth breakage and tooth surface damage.
Tooth surface damage can be subdivided into surface fatigue, abrasive wear and scuffing.
Scuffing may occur in gear teeth when the protective lubricant film breaks down the moment a critical temperature is reached.
Surface damage through scuffing is characterized by moderate wear at the addendum (between pitch circle and top) and dedendum (between pitch circle and root).

3. How rail wheel systems fail. Which statement is untrue?
Fatigue crack formation in rolling contacts is always initiated below the surface.
The cumulative plastic deformation in a highly loaded rail wheel contact is called "Ratcheting".
Lateral traction forces strongly decrease the load capacity in rolling contacts.
The probability of fatigue failure in rolling contacts can be predicted with Hertz theory.

4. How transmission chains fail. Which statement is untrue?
The primary failure mode of transmission chains is wear, i.e. normal wear, abrasive wear, galling wear or corrosive wear.
Fracture by overload seldom appears. Shock overload may cause the link plates or pin to fail without prior deformation.
Fatigue fracture is not a realistic failure mode in transmission chains.
In so-called O-ring chains the critical pin/bush contact is sealed by means of O-rings.

5. How screw joints fail. Which statement is untrue?
The critical area of stress of a standard bolt/nut joint is the effective cross section area of the bolt.
When fatigue crack occurs, it will normally be located in the cross section area at the first engaged thread in the nut.
In some cases so called low height nuts are applied with the same steel grade as that of the bolt. The shear area of the thread in the nut will then be the critical area.
If a screw is turned in a tapped hole of lower steel grade the length of engagement is to be estimated to ensure the reliability of the joint.

6. How journal bearings fail. Which statement is untrue?
Bearings with a soft thin layer are less sensitive to abrasion, adhesion and edge effects from misalignment.
Ceramic bearings have high resistance to abrasive wear but require very good alignment in order to prevent for edge effects.
Scuffing is a typical failure mode for lubricated metal bearings.
Hardened steel shafts sliding in bronze bearings do not need to be lubricated.

7. How drive shafts fail. Which statement is untrue?
Elementary static analysis of strength is applicable to drive shafts that mainly transfer a torque of constant magnitude.
Bending moments even constant of magnitude always result in dynamic stresses.
Drive shafts are generally dynamically loaded (cyclic bending stresses) and thus reliable design requires a fatigue strength approach.
Elementary static analysis of strength is applicable to dynamically loaded drive shafts in the absence of local stress concentrations.

8. How rotary seals fail. Which statement is untrue?
The speed and sealing pressure are often limited by the pv-value of a seal.
The pv-value is a measure for frictional heating.
Rotary lip seals operate in the full film regime. At very low speeds and during running in, mixed lubrication occurs.
A reverse pumping rotary lip seal if correctly mounted produces a pumping action through underneath the seal, thereby improving the lubrication.

9. How power screws fail. Which statement is untrue?
Power screws are subjected to wear. The service life may be limited by axial play and the shear strength of the remaining thread thickness.
The allowable contact pressure at the interfacing thread is limited by seizure.
Steel spindles sliding in bronze nuts do not need to be lubricated.
Long compression loaded spindles must be designed considering buckling.

10. How cam mechanisms fail. The prescribed hardness of a cam and rolling follower should be 500HBW. The measured hardness is 10% less. Calculate the effect of the lower hardness on the load capacity. The load capacity of the line contact formed between the cam and follower is reduced by about