Mounting Sensor Magnets with Tolerance Rings
In today’s automotive, industrial or appliance applications, there is a demand for position accuracy and monitoring. This requires that the sensing device be mounted within the envelope of the design and therefore be adaptable to the application’s design and cost constraints. Therefore they must withstand large temperature changes, shock and vibration, and leave the magnetic field relatively unaffected. Finally they must hold the magnet in the correct position for accurate control or precise measurement and be easy to install.
Tolerance Rings provide many design advantages. The unique wave, formed into the stainless steel strip, provides a force to hold the magnets firmly but gently in place. The configuration of the Tolerance Ring can be adjusted to accommodate the many design constraints providing a spring rate strong enough to hold over a large tolerance range yet soft enough to prevent damage to the magnet material.
The typical material used in Tolerance Rings is 301 stainless steel, which is slightly magnetic due to in process work hardening, which insignificantly affects most magnet applications. Critical applications may require degousing or stress relieving. The 301 stainless is non-corrosive in most environments and will retain its spring properties and hold the magnet over the life of the product. Glue joints can change over the life of the product and fail to hold over time or under high temperatures. Some added design considerations may be necessary if the product will be exposed to severe overloads or shock to prevent the ring / magnet form damage. Direct contact press fits or tight fitted glue joints transmit the loads directly to the magnets that may result in magnet material damage.
The 301 stainless is non corrosive in most environments and will retain its spring properties and hold the magnet over the life of the product. Glue joints can change over the life of the product and fail to hold over timer or with high temperature exposure. Some added design considerations may be necessary if the product will be exposed to severe overloads or shock to prevent the ring / magnet form damage. Direct contact press fits or tight fitted glue joints transmits the loads directly to the magnets that result in magnet material damage.
Many designs require adjustment of the angular position – must line up with the mating component or axial position of the magnets. This is a typical problem due to tolerance stack up of the mating components in the assembly. The air gap between components is important to performance and the Tolerance Ring, unlike the other typical mounting methods, will allow the magnet or magnet holder to be adjusted for either rotational or axial position. Achieving this without Tolerance Rings requires very tight and expensive tolerances on the mating parts and very costly alignment fixtures to control the final assembled position. Care must be taken to assure that all glues or adhesives are fully cured prior to assembly into an enclosure. Out-gassing during the cure can and does damage critical components, magnet wire insulation, etc., if trapped in the assembly envelope. Some further out-gassing may occur during use especially at high temperatures.
If, in the final assembly, the position of the magnets is not correct, the assembly is normally scrapped . Glue and press fits do not allow any adjustment. With the tolerance ring the assembly can be salvaged as the ring allows the magnet to be adjusted into the correct position for proper performance. Better yet it can allow the final position of the magnet to be set during the assembly process to accommodate for the multiple component tolerance stack up.
Today’s devices are made of various materials to make them lightweight, quiet, or even snap together. These materials have different rates of thermal expansion and are exposed to a large range of temperature in storage and operation. Automotive under hood temperatures range form –40F to 350F and higher, causing a massive range of stress on the sensing components. With a direct press fit the magnet material becomes the weak link and can be damaged or the magnet holder can become loose, compromising position control. Glue joints can soften or become brittle as a result of temperature and time, resulting in system failure On the other hand, a Tolerance Ring’s natural spring range allows for the differences in thermal expansion, countering the long term effects of creep and cold flow with softer plastic housing materials.
Assembly is always important with any mounting method. We already discussed the advantages of being able to make adjustments with the use of a Tolerance Ring. Now lets look at the assembly.
Typically it is as easy as:
- Place the Ring into the bore for an AN style ring. The ring is a split ring that easily compresses for ease of insertion and springs open in the bore to be self-retaining. It will not come out if moved to another workstation or even inverted.
- Position the magnet or the magnet holder in the bore so that it nests on the shoulders of the wave.
- Using uniform pressure to keep the parts generally square and concentric, press the magnet or its holder into the Ring.
That it for a perfect fit.
NO Fume
Hoods
NO MSDS
issues
NO special
storage or handling or even gloves
NO shelf
life issues
NO bank
of assembly fixtures to hold parts while curing
NO curing
ovens
NO question
if this batch is fully or properly cured
NO unit
failures - the press force tells you that you have a good assembly
NO scrap
if the position is not exact – adjust it
USA TOLERANCE RINGS
85 Route 31 North, Pennington, New Jersey 08534
phone:609-745-5000; fax:609-745-5012