Secondary shafts: joints breakage

• Splined Shaft: longer portion, on which are mounted and/or inserted most part of other sub-system components;

• Tripods Splined Shaft: shorter portion with two splined ends, the first one needed to engage internally the Splined Shaft, the second one to accom-modate the tripods joint and subsequently the tulips. Note that there are tripods of two different sizes, in order to couple with large and small tulips, according to the C5.14 model.

Red circles in the sketch are meant to highlight the critical spots of the system:

(a) Once a shift, two maintainers stop the test bench in order to tight up the three screw M5x16 used to reinforce the splined connection between Splined Shaft and Tripods Splined Shaft.

(b) Periodically (almost every 6 or 8 months, according to the Machine Ledger), the "SECONDARY SHAFTS" sub-system suddenly stops and maintainers who perform the repair found out the lamellar joint is seriously damaged or even broken, making the secondary shaft’s electric motor not able to transmit the motion any more. Moreover, it frequently happens that broken reed petals are spread all over the shield, causing malfunctions of other nearby components too.

Starting the inspection in June 2021, it came out the issues are common to all test benches and they show up periodically:

(a) Constantly after about 1.5 years from the test bench installation;

(b) After 6 months or not later then 1 year after the elastic joint replacement.

By dedicating a paragraph to each issue, present work will try to find a reliable solution to the problems, giving some hints about the possible corrective actions implementation.

6.2.1 Tripods joint unscrewing

To face the issue, the very first thing to do is to find the potential root cause of the abnormal vibration developed at the tripods joint level, between Splined Shaft and Tripods Splined Shaft. Such vibration makes necessary the tighten of six screws M5x16 (three for each drive shaft), at least three times a day.

After a confrontation with the mechanical specialist of Maintenance Depart-ment, most probable cause is recognized in the wear of the internal splined connec-tion, which cannot be properly hold by three small screws like the ones adopted

by the machine maker: they probably selected M5 screws because of space con-strains, but such small sized threaded elements could be not sufficiently strong to hold the system, so that when the mechanical clearance between splined connec-tion’s teeth increases, vibration dumping on the screws intensify, thus starting the screws unscrewing, and eventually the rise of shear stress.

Present diagnosis is consistent with the issue occurrence: new shafts do not show such a behaviour, while components mounted on test benches for 1 or at least 2 years, start to exhibit the M5 screws breakage, sign of an abnormal shear stress bore by them.

The internal splined teeth wear shows up even though their surface is mechan-ically treated to be strengthen up. Unfortunately, pictures of the worn splined connection are not available, as it would have required the dismounting of the complete sub-system, not compatible with production schedules.

Waiting for more accurate diagnosis, in following paragraphs there will be the description of three possible alternative solutions, to strengthen the connection or to change its shape, in order to stop the abnormal vibration source coming from the joint. The three solutions will be presented in order of "invasivity" and will be shortly commented in final paragraph 6.2.3

BOLT SECURING WASHERS A first and simple solution initially proposed by Thyssenkrupp designer, consists in the insertion of bolt securing washer on the three screw M5x16. The item used is from Nord Lock maker and can be seen in Figure 6.32.

Figure 6.32: Locking washers from Nord Lock

The solution was initially tempted at the beginning of 2020, and the joint looked like the sketch depicted in Figure 6.33a. However, tests run in Thyssenkrupp plant showed an ineffective action of the washers placed on the Tripods Splined Shaft:

being the shaft thermally treated to strengthen the splined connection, washers cannot actually grasp the metal surface on which their are placed, thus making the application useless.

Under Mirafiori Meccanica’s request, Thyssenkrupp proposed a second layout, testing it on Thyssen1 before its delivery to the plant. This time the locking washers are placed on an intermediate spacer, made in mild steel, on which the anti un-screwing components seems to work properly.

(a) First design attempt (b) Second design attempt

Figure 6.33: Bolt securing washers possible solutions

In any case the solution is not believed to be the resolutive one: washers work perfectly on TH1, but as mentioned before, issue will show up later on, along the newest test bench’s life, so that positive results assessed by Thyssen could be not so relevant in the end. Moreover, bolt securing washers might help the screws to stay in place without undergoing shear stress, but if the splined connection suffers of excessive or asymmetric wear, washers would be just a temporary answer to the problem.

SPLINED PROFILE MODIFICATION Actually the used splined profile corresponds to standard DIN 5472 (6x23x28), with straight teeth (Figure 6.34).

As mentioned right above, it should be important to evaluate the type of wear the splined connection shows after 1 or 2 years of operation, so that with respect to the peculiar damage discovered, it would be necessary to apply different corrective actions:

1. uniform wear on both tooth sides: it is just the normal splined connection’s