Feasibility study of a miniaturized magnetorhological grease timing trigger as safety and arming device for spinning projectile

Abstract

A safety and arming (S&A) device keeps the fuze of a projectile unarmed during shipping, handling and storage, while arming the firing section at a proper time by sensing external conditions such as pressure, position, etc. With the increasing need for smaller S&A devices, a miniature design with a compact configuration and high reliability is in demand. This paper proposes a miniaturized timing trigger as the S&A device for a spinning projectile by utilizing the ‘locking’ and ‘unlocking’ properties of magnetorheological (MR) grease with/without the presence of a magnetic field. First, the design and arming mechanism of the timing trigger are introduced, in which the MR grease is locked by a magnetic field generated by two permanent magnets (PMs). Under sufficient firing acceleration, the PMs disengage to unlock the contraction flow of the MR grease, which enables its triggering function. A theoretical analysis was conducted to interpolate the delay time against the geometry of the device, the shear/extensional characteristics of MR grease and the spinning rate of a projectile. A series of tests have been conducted to measure the delay times by tuning the physical parameters, including particle concentration, spinning rate and orifice diameter etc. The experimental results showed that this theoretical model is capable of calculating well the delay time of a MR grease timing trigger.

Effects of Silicone Oil Viscosity and Carbonyl Iron Particle Weight Fraction and Size on Yield Stress for Magnetorheological Grease Based on a New Preparation Technique

“…This interesting phenomenon may be related to variation of CIP size resulting from the alteration of the interparticle contact area and suspension microstructure. Microstructural alterations might change the thixotropy and the yield stress of the suspension under low magnetic field strengths [5,19,28].…”

“…The grease medium was thickened with the increment of SOV, and thus caused an increase in the viscosity of the medium. The increase in viscosity and increase of flow resistance of the suspension was caused by physicochemical interactions leading to the formation of a thicker fiber structure between the medium and CIPs [19,28,38,39]. Thus, the yield stress was strengthened by the production of denser fibrous structure due to the increase of SOV.…”

“…To prevent MR fluid from leaking, the apparatus requires appropriate sealing, thus raising the manufacturing cost [18]. Unlike MR fluid, MR grease, due to its viscosity, has a significant sealing effect and has been applied to a shear-type dampers, thus avoiding the requirement of sealing elements [19,20]. Although grease is used as a suspension medium, leading to high off-state viscosity of the MR grease, the suspension medium does not significantly affect the MR response [5].…”

“…It should be noted that the shear viscosity of MR grease typically decreases with increasing shear rate [11]. This behavior is known as shear thinning and is attributed to the inherent lubrication provided by grease between moving surfaces within high operation velocity ranges [15,19]. Furthermore, MR grease can decrease the loss rate of the lubricating grease and improve the lubrication to reduce friction [22].…”