I4GMMA81

LabWork #2: bolt assemblies part 1

Objective: Highlight the effect of preload on the axial behavior of a bolt

assembly

Lab work #2: bolt assemblies part 1

Content and schedule

1 Description of the test bench (20’)

1.1 Test bench

1.2 Actuators

1.3 Measures

1.4 Accessories

1.5 Preliminary analytical study

2. Work to be done 2.1 Determine the external load (20’)

2.2 Study of the tightening of the assembly (65’)

2.3 Study of the axial behavior of the bolt assembly (55’)

2.4 Study of the SAM torque wrench (20’)

1 Description of the test bench:

This lab work exploits the ER420 test bench described hereafter.

ER420 Test Bench

1.1 Test bench

Le test bench enables to test the axial behavior of a bolt assembly which is composed of a

threaded shaft (tige filetée) and a tubular spacer (entretoise tubulaire). The external axial

load is obtained by a hydraulic jack (vérin hydraulique) with separated pump.

The following pages describe:

� The base of the test bench

� The tubular spacer (entretoise tubulaire)

� The hydraulic jack (vérin hydraulique)

Caution: the friction induced by the seals (joints) in the hydraulic jack induces a loss of

load of about 1300 N.

1.2 Actuators

The loading of the jack (piston) is ensured by a hydraulic hand pump (pompe hydraulique à

main).

Pressure can be lowered by slightly unscrewing the lateral wheel (this induces the liquid to

go back in the pump).

Tightening of the nut (écrou) can be done by using three different torque wrenches.

Dynaflash torque wrench (clé dynamométrique) enables to define precisely the required

tightening torque. When the required torque is reached, a light appears and a sound is

emitted.

Caution: the load has always to be exerted with respect to the arrow.

Attention : il faut toujours forcer dans le sens de la flèche

The Andilog torque wrench shows continuously the torque on its embedded screen.

The SAM torque wrench has an automatic triggering. The required torque is set by

screwing/unscrewing the inside the end of the rod inside the handle. By default the

triggering torque is set to an acceptable value.

1.3 Measures

1) The dial indicator enables to measure an axial displacement.

2) The tightening torque can be set using any of the three torque wrenches.

3) The manometer at the end of the pump shows the pressure on the hydraulic jack.

4) The DELTALAB strain bridge enables to visualize the values of each strain gauge

(jauges de deformation). The On/Off button is located at the back of the device.

Digital indicator

Balance each gauge

Channel selector

On/off gauge factor

Modify the gauge factor

Block the potentiometer

Gauges on the threaded shaft have a gauge factor of K = 2,14. Gauges on the tubular spacer

have a gauge factor of K = 2,10. All the gauges are mounted by opposite pairs on

Wheatstone bridge. Thus the digital indicator exhibits twice the real strain when the correct

gauge factor is selected.

Caution: Set the gauge factor to 1000, in order to directly read in 0,1 mV

Channel 1: longitudinal gauges of the threaded shaft

Channel 2: 45° gauges of the threaded shaft

Channel 3: transversal gauges of the threaded shaft (unexploited)

Channel 4: longitudinal gauges of the tubular spacer

The screw/nut joint (système vis/écrou) induces combined

compression/extension/torsion in the studied system. The longitudinal gauges are not

influenced by torsion. It is thus possible to extract data from channel 1 and 4 to measure the

extension of the threaded shaft and the compression of the tubular spacer.

Extension of the threaded shaft

Gauges of the threaded shaft are glued on a 15 mm diameter.

Surface section is : S = π d2 / 4 = 178 mm2

The axial load is : F = E S ε

With E = 205 000 MPa

F = 205 000 * 178 ε = 36,5 106 ε

As ε is read with a 10-6 factor, F = 36,5 ε when ε is in µm/m and F in N

Digit is the value displayed on the digital indicator

As Digit = 4.24 ε, F = 36,5 Digit / 4.24 = 8,6 Digit

Thus the extension load of the threaded shaft is determined with channel 1 value:

Fv (N) = 8,6 DigitChannel1

Compression of the tubular spacer

The external diameter of the spacer is 40 mm and its inner diameter is 24 mm.

Surface section is thus : S = π (D2 –d

2) / 4 = 804 mm2

Compression load is : F = E S ε

With E = 70 000 MPa

F = 70 000 * 804 ε = 56,3 106 ε

As ε is red with a 10-6 factor, F = 56,3 ε when ε is in µm/m and F in N

Digit is the value displayed on the digital indicator

As Digit = 4.20 ε, F = 56,3 Digit / 4.20 = 13,4 Digit

Finally, the compression load of the tubular spacer is determined with channel 4 value

Fp (N) = 13,4 DigitChannel4

Contrary to Channel 1 and 4 which are sensitive to extension and compression, Channel 2

has been mounted so as to be sensitive to both torsion and extension.

When the threaded shaft is submitted to torsion only, torque is determined with channel 2

value:

T = G π D3 ε / 8 with G = E / ( 2+ 2ν) = 78846 MPa

As Digit = 4.20 ε, : T (N.m) = 0,0249 DigitChannel2

For a combined extension-torsion state in the threaded shaft, the relationship has to be

modified to eliminate the extension effect. The torque of the threaded shaft calculated using

data from Channel 1 and Channel 2.

Cv (N.m) = 0,008 DigitChannel1 - 0,0249 DigitChannel2

1.4 Accessories

� 1 washer (rondelle)

� 3 nuts (écrous)

� 1 leveling pad (Cale de modification du plan de chargement)

� 3 Tubular spacers (Tubes de modification du plan de chargement)

� 1 threaded leveling pad (Cale fileté M16)