6. BOLTING

6.1 General

Ensure on delivery that bolts are in accordance with drawings and specification.

All bolts must be fitted with washers under bolt head or nut, always under the turned part. Fit taper washers at all sloping face connections to ensure all round bearing of bolt head and nut.

    Bolt length must be at least a two full thread projection beyond nut after tightening. In bearing type connections the threaded parts of bolts must not protrude into the bearing members being joined.

    In connections consisting of larger bolt groups always start sequence of tightening from the stiffer part and/or from centre of connection and proceed outwards. On completion of tightening check bolt tensions and repeat the tightening procedure as necessary.

    In structures carrying vibrating machinery, bolts must have either spring washers, lock nuts or similar to prevent loosening or be of the pretensioning type such as HSFG, Huck bolt or similar.

6.2 Installation of bolts

These steps are illustrated in Fig. 14.6.

(a)   Members are brought together and

        holes aligned with drift pins.

(b)   Enough bolts with their appropriate

        washers and load indicators are

        partially tightened.

(c)    Fit bolts into remaining holes.

        Tighten until average gap on each

        load indicator is not more than the

        figure quoted in Table 1. Work

        from the centre of the joint

        outwards towards the free edges.

(d)   Knock out drift pins and replace

        with further bolts and tighten. To

        avoid trapping bolts by joint slip,

        it is important that they are

        tightened before the drift pins are

        knocked out.

6.3 Structural bolts

These fall into two groups:

(a) Black bolts-the majority used for shear and tension purposes.

(b) High strength friction grip bolts.

6.4 Black bolts

(a) Used in holes with 2mm clearance over shank diameter. The two

grades most used in structures



(b) Strength grade designation

This is given by two figures separated by a stop. The first figure

is one-tenth of the minimum ultimate stress in kgf/sq mm. The

second figure is one-tenth of the percentage of the ratio of minimum

yield stress to minimum ultimate.

    Thus '4.6 grade' means minimum ultimate stress of 40 kgf/sq

mm and yield stress 60% of this. It follows that the yield stress is

obtained by multiplying the two figures together to give 24 kgf/sq mm.

    For higher tensile products where the yield point is not clearly

defined, the stress at a permanent set limit is quoted instead of yield

stress.

    The single grade number given for nuts indicates one-tenth of

the proof load stress in kgf/sq mm and corresponds with the bolt

ultimate strength to which it is matched, e.g. an 8 grade nut is

used with an 8.8 bolt. It is permissible to use a higher strength

grade nut than the matching bolt number and grade 10.9 bolts are

supplied with grade 12 nuts since grade 10 steel nut does not appear

in the British Standard series.

    Tighten bearing bolts with podger spanners complying with BS

2583. Ensure that the assembled parts are correctly drawn together

on completion of tightening and any gaps in spring washers (if used)

are fully closed.

    Check that bolts, nuts and washers all match.

6.5 High strength friction grip bolts

(a) Introduction

High strength friction grip (HSFG) bolting is an economic and

efficient method of joining. The procedure is simple, but it is

important that the friction grip principle is fully understood.

    Ensure that adequate instruction is given to erectors. It is

important that they appreciate the difference between using ordinary

black bolts and HSFG Bolts. Using inadequate tools and methods

makes it impossible to obtain correct bolt tension.

(b) The theory

Friction grip depends on tightening each HSFG Bolt to a minimum

shank tension to induce the specified clamping force on the

components being joined.

    This enables the applied load to be carried by friction between

the joint faces rather than relying on the shear strength of the bolts

which would fail to make use of the friction grip principle.

    For the vital friction to be developed, the joint faces must be

clean bare metal or in accordance with the designer's specific design

requirements.

    Examine the steelwork drawings and where HSFG bolts are

specified, check that the joint faces are clean bare metal. Unless

the designer provides a specific waiver these joint faces must NOT

be painted, see Fig. 14.8.

    Even when the applied load is in the direction of the bolt axis,

tightening to a minimum shank tension is still required. The

compression induced in the joining plies by the clamping force

prevents the plies separating and thus there is very little or no

increase in bolt tension so long as the applied load is less than the

bolt pre-load, see Fig. 14.9.



Fig. 14.8 Load held by friction between plates                Fig. 14.9 Load held by friction between plates.


(c) The method

The security of friction grip joints depends on correct tightening

of bolts.

    There are three recognised methods of tightening: torque

control, part turn or turn of nut and direct tension measurement.

    The torque method uses a calibrated torque wrench or a torque

cut-out pneumatic power wrench to give a torque measurement.

It should be appreciated that torque only measures resistance to

turning. Because of varying conditions of nut and bolt threads,

relationship between torque and tension induced is dependent on

those particular conditions.

    When threads are dry, dirty, damaged or lacking in lubrication,

friction will be high and, at a given torque setting, bolt tension

will be low. If threads are well lubricated and free running, tension

induced in a bolt at a similar torque setting will be high. A scatter

of ± 40% can readily be obtained by experimenting with bolts

in different conditions using a pre-set torque wrench and a load

meter.

    The part turn method was devised to overcome concern caused

by these factors. The principle behind this system is to tighten bolts

into or approaching yield by first snug tightening and then applying

an additional part turn, the amount depending on the grip length

of the bolt. This method needs a great deal of operator reliability

and supervision. It can only be inspected by close observation

during tightening or by reverting to the already suspect accuracy

of the calibrated torque wrench method. Note that the British

Standard for the use of higher grades states that the part-turn method

is not permitted.

    The direct tension method gives a direct indication on the load

induced along the axis of the bolt. A load indicating device is fitted

into the grip of the bolt assembly eliminating the unpredictability

of the torque measurement. The Standards specify tension not

torque.

The 'Coronet' load indicator is a typical direct tension

measuring device comprising a specially hardened washer with

protrusions on one face. As the bolt is tightened, the protrusions

are flattened and the gap between the load indicator face and the

bolt head is reduced. The gap is measured by a feeler gauge

and, at the specified gap given in Table 1, the shank

tension will not be less than the minimum load specified in the

appropriate Standard.


 

Fig. 14.11 High strength friction grip bolt with 'CORONET' load indicator.

    Sometimes it is more convenient to fit the 'Coronet' load

indicator under the nut. Place a special nut face washer of similar

hardness to the bolt head between the protrusions of the load

indicator and the nut. Then follow the same tightening procedure.

See Fig. 14.12.

    If the bolt head has to be turned to tension this assembly, put

an additional flat round washer under the bolt head.

In turning the bolt head with the load indicator under the head,

use a nut face washer assembled in the same way but underneath

the head. No further washers are required in this application.

    With taper flanges, use taper washers as shown in Fig. 14.13

and 14.14.

 

Fig. 14.12 Use of nut face washer.


Fig. 14.13 Use of taper washer (1).


Fig. 14.14 Use of taper washer (2).

 

 

(d) The components

Bolts, nuts, load indicators and washers are clearly marked

according to grade. Check that bolts, nuts and washers match each

other. (See Fig. 14.15).

(e) Corrosion

HSFG Bolts, nuts, washers and load indicators will corrode if not

properly protected. Keep in a clean, dry and well ventilated store.

Only issue from the stores the number required for immediate

installation so that none lie about the site deteriorating.

Paint soon after tightening and seal gaps with paint or other

approved material. This is vital in marine or other corrosive

atmospheres.

Metallic coatings applied by bolt manufacturers give only

temporary protection during storage and installation -early painting

will still be required. Do not store plated items in hessian bags

as they absorb moisture and damp conditions lead to rapid

deterioration of the zinc or cadmium. Research shows that

susceptibility to stress corrosion and hydrogen embrittlement

increases with tensile strength. In certain conditions a metallic

coating may reduce resistance to stress corrosion, and the specifier

should take account of the site environment in deciding the

suitability or otherwise of a coating. This applies particularly to

the higher strength items, viz washers, higher strength bolts and

load indicators.

(f) The tools

Hand wrenches may be satisfactory for tightening small diameter

bolts but use power tools or torque multipliers above size M22.

Impact wrenches must have adequate capacity to tighten bolts

within 15 seconds. Prolonged impacting can damage the bolt

assembly or fracture the bolt

    Choose a tool with a torque output more than that required for tightening the largest bolt for which it will be needed thus making some allowance for loss of performance due to wear, air leakage etc and to help overcome the energy absorbed by higher than usual thread friction or 'springy' joints.