Is this the start of something small?
TWI Bulletin, June 1986
by Clive King and Richard Sproulle
Clive King is Head of the Control Engineering Department and Richard Sproulle, BSc, is a Senior Design Engineer in the same department.
A novel design of friction welding head opens new possibilities for the process, including easier access to restricted areas and robotisation.
Friction welding provides a reliable method of joining a range of materials and the process is increasingly being considered for joining metal studs to plate in 'on-site' applications.
Designs and machines for friction welding have for many years been produced both by The Welding Institute* and by some of its Member companies. In general the designs have followed the principle of having a prime mover for rotation and a hydraulic or mechanical force system. The new design concept described in this article is an attempt to improve portability to help overcome problems of placing studs in restricted access and working areas and also to enable FW machines to be used in clean conditions, for example in nuclear plants, where hydraulic fluid is not permissible.
*King C: 'Products for industry and research - the Control Engineering Department at work'. Welding Institute Research Bulletin 1985 26 (12) 400-408.
A new concept
The most common method of friction welding uses simple rotary motion of one axially symmetric component with respect to a stationary component under an applied axial load to develop heat at their common interface ( Fig.1).
Fig.1. Principles of friction welding:
a) Rotation begins;
b) Initial contact at reduced load;
c) Heat generated at interface as load is increased;
d) Final forge force applied to consolidate joint
The prototype friction welding head (patent applied for) described here ( Fig.2 and front cover) incorporates an automatic self-regulating friction load controlled, via a cam profile, by the rotational rundown speed of weighted arms. The operation of the machine is simple. For a given stud materialsize (up to 12mm diameter at this time) the cam shape is selected and the stud to be welded is gripped firmly against rotation. The rotational welding speed is then obtained by various methods, for example electric or pneumatic motor.The power required for rotation is small as the starting torque of the head is low when the arms are at minimum radius, in the start position.
Fig.2. Prototype two arm friction welding head:
a) Stationary;
b) At maximum speed
Operating principle
At the desired speed of rotation, the stud and plate to be welded are brought into contact. The drive can then be declutched or switched off so that the rotating head begins to slow down. As the rotational speed decreases, the offsetting pre-compression of the spring stack is relaxed and the load applied between the stud and plate increases. This action progressively develops until, when rotation ceases, the load on the components reaches a maximum. With this simple 'flying bob' governor system, the axial force developed is a function of the square of the rotational speed, the effective radius of the bobweight and the angle of the crank arm to the centre axis.
In a simple case, where friction is neglected, the axial force is given by:
Where ω is the angular velocity, M 1 is the total mass of the bobweights and M 2 is the total mass of the arms between the bobweights and the fulcrum. The system is illustrated in Fig.3.
Fig.3. Detail of lever arm/spring mechanism
If the radius of contact L 2 does not change significantly with arm angle, α, and as the masses are constant for any given head configuration, the 'governor' axial force at a given speed will increase with α as shown in Fig.4a. The spring reaction shown is not speed related and is solely dependent on the physical dimensions of the welding head, so that at any given speed an equilibrium point will exist between the curves of governor force and spring reaction so as to give 'stored force' against arm angle.
The graphs shown in Fig.4b and c are developed for cams where L 2 changes with arm angle α, cam C showing the most marked change. The values used to calculate these curves are shown.
Fig.4. The change in spring force and governor force with angle α at different rotational speeds for various designs of cam:
a) Cam A; L 2 = 0.020m (min), 0.024m (max); ω = 175 rad/ sec (min), 275 rad/sec (max); L 1 = 0.055m; R = 0.045m; M 1 = 0.21 kg; M 2 = 0.25 kg;
b) Cam B; L 2 = 0.011m (min), 0.025m (max); ω = 125 rad/sec (min), 295 rad/sec (max); L 1 , R, M 1 and M 2 as Fig.4a;
c) Cam C; L 2 = 0.011m (min), 0.024m (max); ω = 125 rad/sec (min), 275 rad/sec (max); L 1 , R, M 1 and M 2 as Fig.4a.
The resultant axial forces for different cam shapes and the corresponding angular velocities are shown as functions of time in Fig.5.
Fig.5. Change in axial force and corresponding decay speed as a function of time for the three cams A, B and C
A maximum force of 5kN is obtainable on decelerating from 300 rad/sec (2900rpm) using the welding head shown in Fig.6.
Fig.6. Two arm friction welding head (drive is not shown)
A portable system
This compact design, where the applied force is derived from the rotational speed of the friction head, makes friction welding a more versatile process. Thus, instead of considering a friction welding machine as being fixed it canbe considered as a portable system to make welds in situ, e.g. by attaching it to a robot. Clamping can be accomplished in many ways.
Using basic prototype heads, welds have been made in steel and plastics ( Fig.7). Further development towards a simple automatic friction welder where only the initial wind-up speed need be controlled can be achieved. Member companies wishing to discuss the potential of this system and its furtherdevelopment are invited to contact either of the authors at Abington.
Fig.7. Completed friction welds. From left to right: 070M20 carbon steel; cross section of steel joint; and weld in polymethylmethacrylate
