Apr 02 - Heli-Coil Tech Bulletin 898-1

Purpose:

The purpose of this test was to compare the effects of galling on standard tapped hole assemblies and assemblies prepared using Heli-Coil Heli-Tronic gall-resistant inserts. Although tests using standard Heli-Coil inserts of 304 stainless steel (AMS7245) showed some resistance to galling due the work hardening of the material, the presence of torque ‘scatter’ indicated the possibility for inconsistent readings. The Heli-Tronic insert tests showed no indication of scatter in any tests as indicated by the graphs in the following pages.

Procedure:

1. #10-32 standard tapped hole blocks were prepared in 316 grade stainless steel material. All tapped holes were electro-polished.

2. #10-32 STI holes were prepared to Heli-Coil specifications in aluminum material. Heli-Coil Heli-Tronic inserts were installed and tangs removed.

3. Tests were performed using both plain and chemically-polished 316 stainless steel screws

4. Torque tests were conducted at 0, 2.2 and 4.4 degrees from perpendicular (See Diagram).

5. Screw insertion torque and friction were recorded for each set of samples.

TEST RESULTS:

Standard Tapped Holes

• Measurable torque required to rotate screws increased dramatically with an increase in the installation angle. Total friction increase as high as 77% over perpendicular installation.

• Installation torque as measured increased as much as 77%.

Installed Heli-Tronic Insert Assembly

• Measurable friction forces were measured from 17% to 44% lower than the comparable standard tapped holes.

• With increased installation angle, friction forces increased at much lower rate than comparable as machined holes.

• Installation torque as measured, yielded torques as much as 40% lower than the comparable as machined assembly.

CONCLUSION:

Based on the testing completed, Heli-Coil Heli-Tronic screw-thread inserts provide superior gall-resisting characteristics compared to standard tapped holes. Heli-Coil Heli-Tronic inserts also provide increased compensation for mis-alignment of both assemblies and fasteners. In addition, because of the reduction in friction, Heli-Tronic inserts provide a more consistent, true clamping torque during fastener assembly.

Any additional questions should be forwarded to Heli-Coil Applications Engineering at (203) 830-3238 or (203) 830-3264. (Fax 203-778-8409)

Figure 1 As-Machined 316 Screws into Electropolish 316 Test blocks without Helicoils, Correct Screw and Test Block alignment

Figure 2 As-Machined 316 Screws into Electropolish 316 Test blocks without Helicoils, Induced Galling -- 2.2 degree Screw and Test Block Misalignment

Figure 3 As-Machined 316 Screws into Electropolish 316 Test blocks without Helicoils, Induced Galling – 4.4 degree Screw and Test Block Misalignment

Figure 4 Chemical Polished 316 Screws into Nitronic 60 Helicoils, Correct Screw and Test Block alignment

Figure 5 Chemical Polished 316 Screws into Nitronic 60 Helicoils, Induced Galling -- 2.2 degree Screw and Test Block Misalignment

Figure 6 Chemical Polished 316 Screws into Nitronic 60 Helicoils, Induced Galling -- 4.4 degree Screw and Test Block Misalignment

Figure 8 Chemical Polished 316 Screws into Electropolished 316, without Helicoils Induced Galling --2.2 degree Screw and Test Block Misalignment

Heli-Coil

Division of Emhart Teknologies

Shelter Rock Lane

Danbury, CT 06810

Heli-Coil Heli-Tronic Inserts meet material specification UNS S21800.

Testing performed by

Center for Tribology, Inc., 625A Clyde Avenue, Mountain View, CA 94043.

Tel: (650) 928-2387, FAX: (650) 526-3830, E-Mail: www.cetr.com

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