Answer: ACME screws offer several advantages over ball screws including low cost, self-locking capability, high load capacity, resistance to contamination, low noise, design versatility and smaller size.
ACME screws are typically half the cost of similarly sized ball screws making them an excellent value proposition. If your application calls for a self-locking screw, then standard ACME screws are an ideal choice. A self-locking screw will not turn the nut under a thrust load and a good rule of thumb for self-locking is to keep the lead angle under 5 degrees.
The static load capacity of an ACME screw and nut assembly is greater than ball screw assemblies of similar size and material. The dynamic load capacity of an ACME screw depends on the pressure-velocity rating of the mating nut material. At low speeds, the dynamic load capacity of an ACME screw is generally greater than ball screws but less at higher speeds.
ACME lead screws can handle higher levels of contamination than ball screws due to increased internal clearances. As a result, lead screws can to pass larger debris such as metal chips and are less susceptible to dust and dirt that can “gum-up” ball bearings. ACME screws can also be used in washed-down environments which are detrimental to ball screw operation.
Operating noise levels of ACME screws is considerably less than ball screws since they have no internal moving parts. Ball screws generate higher noise levels due to ball bearings circulation within nut.
ACME lead screws offer greater versatility than ball screws because they are not restricted to a standard nut design. ACME screws are easily customized to fit specific applications whereas it is cost prohibitive to redesign and retool new ball screw nuts. For example, lead screws can easily be configured with high helix leads to achieve fast traverse rates which are not generally available on ball screws.
Since ball screw nuts require bearings and circulation tubes, the nut is generally larger than comparable ACME nuts which is often a consideration where packaging space is limited.
Ball screws are more efficient than standard lead screws owing to their lower coefficient of friction. Ball screws rely on a rolling motion which generates lower friction force than the sliding motion employed by lead screws. The efficiency of ball screws is generally in the 90% range whereas standard lead screws range from 20% to 40%. Lead screw efficiency can be improved to the 60%-70% range by adding additional thread starts; however, self-locking capability will be lost.
Ball screws are generally more precise than standard lead screws assemblies due to reduced backlash. That said; there are anti-backlash nuts available for lead screws that help minimize the differences.
For more help with applications, contact Rempco.