INTRODUCTION – ISO 10300 series

When ISO 10300:2001 (all parts) became due for its first revision, the opportunity was taken to include hypoid gears, since previously the series only allowed for calculating the load capacity of bevel gears without offset axes. The former structure is retained, i.e. three parts of the ISO 10300 series, together with ISO 6336-5, and it is intended to establish general principles and procedures for rating of bevel gears. Moreover, ISO 10300 (all parts) is designed to facilitate the application of future knowledge and developments, as well as the exchange of information gained from experience.

INTRODUCTION – ISO 10300-3:2023

In view of the decision for ISO 10300 (all parts) to cover hypoid gears also, it was agreed to include a separate clause: “Gear tooth rating formulae — Method B2” in this document, while the former methods B and B1 were combined into one method, i.e. method B1. So, it became necessary to present a new, clearer structure of the three parts, which is illustrated in ISO 10300-1:2023, Figure 1.

NOTE: ISO 10300 (all parts) gives no preferences in terms of when to use method B1 and when to use method B2.

Failure of gear teeth by tooth root breakage can be brought about in many ways; severe instantaneous overloads, excessive macropitting, case crushing and bending fatigue are a few. The strength ratings determined by the formulae in this document are based on cantilever projection theory modified to consider the following:

— compressive stress at the tooth roots caused by the radial component of the tooth load;

— non-uniform moment distribution of the load, resulting from the inclined contact lines on the teeth of spiral bevel gears;

— stress concentration at the tooth root fillet;

— load sharing between adjacent contacting teeth;

— lack of smoothness due to a low contact ratio.

The formulae are used to determine a load rating, which prevents tooth root fracture during the design life of the bevel gear. Nevertheless, if there is insufficient material under the teeth (in the rim), a fracture can occur from the root through the rim of the gear blank or to the bore (a type of failure not covered by this document). Moreover, it is possible that special applications require additional blank material to support the load.

Surface distress (pitting or wear) can limit the strength rating, either due to stress concentration around large sharp cornered pits, or due to wear steps on the tooth surface. Neither of these effects is considered in this document.

In most cases, the maximum tensile stress at the tooth root (arising from bending at the root when the load is applied to the tooth flank) can be used as a determinant criterion for the assessment of the tooth root strength. If the permissible stress number is exceeded, the teeth can break.

When calculating the tooth root stresses of straight bevel gears, this document starts from the assumption that the load is applied at the tooth tip of the virtual cylindrical gear. The load is subsequently converted to the outer point of single tooth contact. The procedure thus corresponds to method C for the tooth root stress of cylindrical gears (see ISO 6336-3[1]).

For spiral bevel and hypoid gears with a high face contact ratio of ε_vβ > 1 (method B1) or with a modified contact ratio of ε_vγ > 2 (method B2), the midpoint in the zone of action is regarded as the critical point of load application.

The breakage of a tooth generally means the end of a gear's life. It is often the case that all gear teeth are destroyed as a consequence of the breakage of a single tooth. A safety factor, S_F, against tooth root breakage higher than the safety factor against damage due to macropitting is, therefore, generally to be preferred (see ISO 10300-1).

SCOPE

This document specifies the fundamental formulae for use in the tooth root stress calculation of straight and helical (skew), Zerol and spiral bevel gears including hypoid gears, with a minimum rim thickness under the root of 3.5 m_mn. All load influences on tooth root stress are included, insofar as they are the result of load transmitted by the gearing and able to be evaluated quantitatively. Stresses, such as those caused by the shrink fitting of gear rims, which are superposed on stresses due to tooth loading, are intended to be considered in the calculation of the tooth root stress, σ_F, or the permissible tooth root stress σ_FP. This document is not applicable in the assessment of tooth flank fracture.

The formulae in this document are based on virtual cylindrical gears and restricted to bevel gears whose virtual cylindrical gears have transverse contact ratios of ε_vα < 2. The results are valid within the range of the applied factors as specified in ISO 10300-1. The bending strength formulae are applicable to fractures at the tooth fillet, but not to those on the active flank surfaces, to failures of the gear rim or of the gear blank through the web and hub.

This document does not apply to stress levels above those permitted for 10³ cycles, as stresses in that range can exceed the elastic limit of the gear tooth.

NOTE: This document is not applicable to bevel gears which have an inadequate contact pattern under load.

The user is cautioned that when the formulae are used for large average mean spiral angles (β_m1 + β_m2)/2 > 45°, for effective pressure angles α_e > 30° and/or for large facewidths b > 13 m_mn, the calculated results of this document should be confirmed by experience.