8/8/2019 Tolerancing of Screw Threads

http://slidepdf.com/reader/full/tolerancing-of-screw-threads 1/4

Tolerancing of Screw Threads

It was previously stated that an essential principle is that the actual

profiles of both the nut and bolt threads must never cross or transgress

the theoretical profile. Practically, to make a thread, tolerances must beapplied to ensure that this essential principal always applies.

Tolerancing of screw threads is complicated by the complex geometric

nature of the screw thread form. Clearances must be applied to thebasic profile of the threads in order that a bolt thread can be screwed

into a nut thread. For the thread to be made practically there must be

tolerances applied to the main thread elements.

Usually nut threads have a tolerance applied to the basic profile so that

it is theoretically possible for the nut thread profile to be equal to the

theoretical profile. Bolt threads usually have a gap between the basic

and actual thread profiles. This gap is called the allowance with inch-based threads and the fundamental deviation with metric threads. The

tolerance is subsequently applied to the thread. Since for coated threads

the tolerances apply to threads before coating (unless otherwise stated),

the gap is taken up by the coating thickness. After coating, the actual

thread profile must not transgress the basic profile of the thread.

A full designation for a metric thread includes information not only on

the thread diameter and pitch but also a designation for the thread

tolerance class. For example a thread designated as M12 x 1 - 5g6g

indicates that the thread has a nominal diameter of 12mm and a pitch of 

1mm. The 5g indicates the tolerance class for the pitch diameter and 6g

is the tolerance class for the major diameter.

8/8/2019 Tolerancing of Screw Threads

http://slidepdf.com/reader/full/tolerancing-of-screw-threads 2/4

A fit between threaded parts is indicated by the nut thread tolerance

designation followed by the bolt thread tolerance designation separated

by a slash. For example: M12 x 1 - 6H/5g6g indicates a tolerance class

of 6H for the nut (female) thread and a 5g tolerance class for the pitch

diameter with a 6g tolerance class for the major diameter.

A tolerance class is made up of two parts, a tolerance grade and a

tolerance position.

A number of tolerance grades have been established for the pitch and

crest diameters (the crest diameter is the minor diameter in the case of anut thread and the major diameter in the case of a bolt thread.

Tolerance grades are represented by numbers, the lower the number thesmaller the tolerance. Grade 6 is used for a medium tolerance quality

and a normal length of thread engagement. Grades lower than 6 are

intended for fine tolerance quality and/or short lengths of thread

engagement. Grades higher than 6 are intended for coarse tolerance

quality and/or long lengths of thread engagement.

8/8/2019 Tolerancing of Screw Threads

http://slidepdf.com/reader/full/tolerancing-of-screw-threads 3/4

There are:

5 tolerance grades (grades 4 to 8) available for the minor diameter of 

the nut thread.

3 tolerance grades (grades 4,6 and 8) for the major diameter of the bolt

thread.5 tolerance grades (grades 4 to 8) for the pitch diameter tolerance of the

nut thread.

7 tolerance grades (grades 3 to 9) for the pitch diameter tolerance of the

bolt thread.

Tolerance positions are indicated by letters, upper case letters for nut

threads and lower case letters for bolt threads. The tolerance position is

the distance of the tolerance from the basic size of the thread profile.

For nut threads there are two tolerance positions, H with a zero

fundamental deviation (distance of the tolerance position from the basic

size) and G with a positive fundamental deviation.

For bolt threads there are four tolerance positions, h has a zero

fundamental deviation and e, f, and g negative fundamental deviations.

(A positive fundamental deviation indicates that the size for the thread

element will be larger than the basic size. A negative fundamental

deviation indicates that the size for the thread element will be smaller

than the basic size.

8/8/2019 Tolerancing of Screw Threads

http://slidepdf.com/reader/full/tolerancing-of-screw-threads 4/4

One practical problem that is often encountered is what threads

tolerance to apply to a tapped hole. The standard tolerance classes of 6g

for the bolt thread and 6H for the nut thread are typically included on a

drawing as default. A problem that sometimes occurs is that on long

thread engagements (that are frequently used for tapped holes in softmaterials) there can be interference between the nut thread and the

screw thread as the screw is rotated into the tapped hole. There can be a

slight mis-match in the thread pitch between the internal thread and the

external thread necessitating a wrench to rotate the fastener to the

bottom of the thread i.e. it can't be freely rotated. The standard

tolerance classes apply strictly only when a relatively short length of 

engagement is used (such as with a nut which is typically 0.8d where dis the thread size). The pitch diameter tolerance must be able to

accommodate pitch and flank angle errors which can sometimes onlybe done by changing the tolerance position say from a H to a G for the

internal thread (since standard screws - 6g - are wished to be used).

Failure to change the tolerance position can result in thread seizure and

damage especially if high speed tools are being used for the tightening

process.