AGMA 91FTM9-1991 pdf free.Rerating Damaged Naval Ship Propulsion Gears.
Field dressing damaged gear teeth usually involves grinding or stoning of f any proud metal and removing any crack indications1 if any, so as not to leave any stress risers or local high spots which will be subject to increased contact load when put back into service. Figure 1 shows the smoothed surfaces left after such a cleanup. In extreme cases it may be necessary to completely machine off a section of tooth flank from ever)’ tooth around the gear, especially if rotor balance is critical. In addition to smoothing the fracture surface and associated upset metal, an area of tooth flank adjacent to the damaged edg. will be relieved slightly to restore the end relief necessary in large face width gears to avoid end loading.
After cleaning up the damaged teeth, the results are measured in place and documented for analysis. Due to the need to get the equipment back into service quickly, and the high cost of removing the g.ars from the ship for more accurate results, these measurements are typically done with the aid of hand held scales and reported with an accuracy of p.rhaps plus or minus 1/16 inch. Additional information is gathered from the operators and operating logs in order to understand the conditions surrounding the failure, and samples of pieces broken of f are used for analysis of the cause of failure. Casts of the damaged area are often used to record the damage or final configuration, or both.
If foreign satt•r can be identified, the unit will likely be put back into service based on the assumption that this is the only cause, and a follow up investigation will be persued as confirmation only. Otherwise, a complete failure investigation will be done before attempting to rerate to the full remaining capability of the unit.
TOTAL LENGTH OP CONTACT
Rerating the unit after damage is baBed on an examination of the total length of contact, L. The quickest method would be to calculate the stress as if all the teeth are damaged exactly lik, the worst tooth, both in length of tooth lest and axial location of the damage. If one further simplifies the problem to ignore the effects of the actual axial location of damage, a recalculation with a simple reduction of face width yields the rsult. This is adequate for wide face gears and when the damage is in fact in a circumferential band par, or if the damage to all teeth is confined to the axial extent of the damag on the most damaged tooth.However,fol helical gears with perhaps five or more teeth in contact, this approach will be overly conservative if only one tooth is damaged or not conservative enough if damage to several teeth occurs in one axial strip, with all damaged sections passing through mesh at the same time. Also, in some cases,the axial location of the damage band may be significant to the value of Lmin, the minimum instantaneous total length of contact, after damage.AS discussed later,Lmin forms the basis for rerating the gear set.AGMA 91FTM9 pdf free download.